The study presented in the paper was aimed at investigating sorption properties of milk-replacing formulas for infants. The scope of the study covered the evaluation of water content and activity, solubility and content of “total” HMF. Sorption isotherms were plotted over water activity range of aw = 0.07-0.98, then they were subjected to BET transformation over water activity range of aw = 0.07-0.44. Further determinations were conducted for the volume of the monomolecular layer Vm and water activity corresponding to it, as well as for the specific surface of adsorption. The course of sorption isotherms plotted for water vapour of the infant formulas examined was consistent with that of type II isotherms acc. to Brunauer’s classification. Over the water activity of the medium aw = 0.33-0.44, they were characterised by an interruption of curve continuity, expressed by a diminished level of equilibrium humidity of the products analysed. The differences observed in the size of the monomolecular layer were likely to be determined by the content of hydrophilic constituents in the formulas. Based on the determined value of the energy constant C, parameters of the BET equation were found to be burdened with a gross error.