Densities at 25 °C of aqueous mixtures of urea, formamide, N-methylacetamide, dimethylacetamide, acetyl-alanine-methylamide, acetone, and butanone have been measured, and specific volumes, partial volumes, and excess volumes of the mixtures are evaluated. An inflection point of the specific volume, corresponding to a maximum of the partial volume of water, is observed for mixtures of the substances containing aliphatic groups. The excess volumes are all negative, but it is tentatively concluded that the contribution to the excess volumes due to interactions between the aliphatic groups and water is positive. This effect is discussed with particular reference to protein denaturation. The interaction between aliphatic groups and water is further discussed in an appendix to the paper. It is suggested that more than 15 water molecules may interact co-operatively with the non-polar groups, and that the probability that a given group takes part in this interaction is less than one-half.