Composition and compressibility characteristics of Mexico City clay were investigated using samples with natural water contents in the range of 421–574%. The composition study included scanning electron microscopy, X-ray diffraction and chemical analyses, as well as measurements of physical properties. Compressibility characteristics were investigated by means of one-dimensional consolidation tests run on specimens in the following states: natural undisturbed, artificially sedimented, remoulded at natural water content, remoulded-aged, and hydrogen peroxide treated. The primary objective was to study the secondary compressibility of Mexico City clay with special reference to the effects of previous secondary compression and thixotropic hardening. Compositional analysis indicated 5–10% sand-sized particles, most of which are calcareous ooliths (the remainder of this size fraction is a suite of heavy minerals); 55–65% of the whole sample is composed of silt-sized siliceous microfossils, mostly diatoms; 20–30% is composed of clay-sized particles, of which an estimated 10% are smectite and the rest biogenic and volcanogenic silica; the remaining 5–10% is organic matter. Based on test results it was hypothesized that the smectite in Mexico City clay is interlayered with hydroxide complexes of aluminium, iron and possibly magnesium. Primary and secondary compressibilities are interrelated. During sustained loading secondary compression and thixotropic hardening take place simultaneously. After sustained secondary compression there is a decrease in primary as well as secondary compressibility for pressure increments less than the reserve consolidation pressure. Thixotropic hardening, however, increases the viscous resistance, and thereby decreases the amount of compression during primary consolidation, but increases the rate of secondary compression. The structural resistance which develops during sustained secondary compression decreases the rate of primary consolidation during the subsequent increment and results in computed permeabilities which are less than the true permeability of the soil fabric. Les caractéristiques de la composition et compressibilité de l'argile de Mexico ont été examinées en utilisant des échantillons ayant une teneur en eau naturelle de 421% à 574%. L'étude de la composition a compris un examen au microscope électronique, la diffraction aux rayons-X et des analyses chimiques, aussi bien que des mesures de propriétés physiques. Les caractéristiques de compressibilité ont été examinées à l'aide d'essais de consolidation à une dimension sur des échantillons dans les conditions suivantes: naturel intact, déposé artificiellement, remanie à la teneur en eau naturelle et traités au peroxyde d'hydrogène. Le premier objectif était d'étudier la compressibilité secondaire de l'argile de Mexico avec une référence spéciale aux effets de compression secondaire préalables et de durcissement thixotropique. L'analyse de la composition a indiqué 5% à 10% de particules de la taille de sable, dont la plupart sont des oolithes calcaires (le reste de cette fraction est un ensemble de minéraux lourds); 55% à 65% de tout l'échantillon sont des microfossiles siliceux, pour la plupart des diatomées, de la taille du limon; 20% à 30% sont composés de particules de la taille de l'argile dont une estimation de 10% est du smectique et le reste est de la silice biogénique et volcanique. Les 5% à 10% restant sont de la matière organique. Basé sur des résultats d'essai, on a fait l'hypothèse que le smectique dans l'argile de Mexico est intercallé de couches de complexes de l'hydroxide d'aluminium, de fer et peut-être de magnésium. Il y a un rapport entre les compressibilités primaires et secondaires. Pendant un long chargement, des compressions secondaires et durcissements thixotropiques ont lieu simultanément. Une compression secondaire soutenue diminue la compressibilité primaire aussi bien que secondaire pour des augmentations de pression inférieures à la pression de consolidation de réserve. Cependant, le durcissement thixotropique augmente la résistance visqueuse, et par là diminue l'importance de la compression pendant la consolidation primaire, mais augmente le degré de compression secondaire. La résistance structurale qui se développe pendant la compression secondaire soutenue, diminue le degré de consolidation primaire pendant l'augmentation subséquente, et aboutit à des perméabilités estimées, qui sont inférieures à la perméabilité réelle de la matière du sol. Composition and compressibility characteristics of Mexico City clay were investigated using samples with natural water contents in the range of 421–574%. The composition study included scanning electron microscopy, X-ray diffraction and chemical analyses, as well as measurements of physical properties. Compressibility characteristics were investigated by means of one-dimensional consolidation tests run on specimens in the following states: natural undisturbed, artificially sedimented, remoulded at natural water content, remoulded-aged, and hydrogen peroxide treated. The primary objective was to study the secondary compressibility of Mexico City clay with special reference to the effects of previous secondary compression and thixotropic hardening. Compositional analysis indicated 5–10% sand-sized particles, most of which are calcareous ooliths (the remainder of this size fraction is a suite of heavy minerals); 55–65% of the whole sample is composed of silt-sized siliceous microfossils, mostly diatoms; 20–30% is composed of...