The reactions of monolayers of cellulose, spread from Cadoxen solvent, with a variety of solutes in the aqueous phase have been studied by measurement of film expansion, compressibility, and viscosity, with a view to elucidating the mechanism of dye adsorption by cellulose. Refractometry measurements of bulk solutions have also been used. Phenol has no effect on the film, but urea expands it. Cationic dyes, e.g., Methylene Blue, associate weakly with the monolayer to give a mixed film of high compressibility. Anionic dyes slightly expand the film, without change in its compressibility or viscosity. This expansion is almost identical for monoazo-dyes with little affinity for cellulose in bulk, as it is for bisazo-dyes, with similar molecular width, but of high cellulose affinity. A direct dye with a wider molecule causes greater film expansion, approximately proportionate to this increased width. This evidence suggests face-to-face association of cellulose and anionic dye molecules parallel to the cellulose chain. Refractometry, but not the other tests, reveals weak complexing, probably of an acid–base type, between cellulose and amino-groups in dyes. This is the only dye–cellulose force identified. Dye adsorption by cellulose in bulk is explained by this and other recent work, e.g., the known tendency of the dye to escape from aqueous solution, causes it to concentrate at the cellulose–water interface. There it becomes aligned parallel to the cellulose chains possibly by hydrophobic bonding with glucosidic > CH groups. It then associates to form multilayers or three-dimensional aggregates. This process occurs more readily with long narrow planar molecules and is assisted in technical practice by the presence of excess of neutral inorganic salt in the dye-bath.