Department of Chemistry, Panjab University, Chandigarh Manuscript received 17 May 1974; accepted 8 April 1975 Adsorption isotherms of water vapour by casein and its major fractions obtained at 30° have been analysed by B.E.T., Harkins and Jura and Frankel, Halsey and Hill equations. The values for monolayer capacity as obtained by the first two equations and as read from the knee of the isotherm itself are in good agreement with one another. The mean diameter of casein particles, as calculated from specific surface area value, is in fair agreement with the value obtained by the use of electron microscope as reported in the literature. The correctness of the monolayer capacity is also supported by sudden change in thermodynamic functions, namely, free energy, differential heat and entropy of adsorption. The formation of multimolecular layers at r.v.p.'s ≥0.6 is supported by Frankel, Halsey and Hill plots. The water sorbed at r.v.p. ≤ 0.6 does not freeze even at -40° ≤, and may be taken as held tightly to the surface.