Using the Picker flow microcalorimeter, molar excess enthalpies (hE) and molar excess heat capacities (cEP) have been obtained at 25°C for nonane isomers of different degrees of chain branching mixed with n-octane, and for 2,2,5-trimethylhexane and 3,3-diethylpentane (3,3-DEP) mixed with other normal alkanes and with cycloalkanes. As the degree of steric hindrance within the nonane isomer is increased, hE with n-octane changes from positive to negative and cEP from negative to positive. Values of hE are also anomalously negative for 3,3-DEP + n-Cn with n⩽ 14 and for 3,3-DEP + cyclopentane. Correspondingly, cEP is anomalously positive for 3,3-DEP + n-C6 and for 3,3-DEP + cyclopentane. The results are interpreted in terms of two contributions: (1) destruction of orientational order in the pure n-alkane giving hE > 0, cEP < 0, and (2) a “condensation” of the more freely moving molecule, e.g., n-Cn or cyclopentane onto the sterically-hindered 3,3-DEP resulting in a restriction of movement giving hE < 0, cEP > 0, i.e., a creation of order in the solution.