The moisture sorption isotherms of three different infant formulas (denoted as P1, P2 and P3) that contained almost the same amount, but different types, of protein were analysed and their physicochemical changes characterised. Although P3 showed a different trend as compared to both P1 and P2, all the powders had the highest moisture content at 0.428 a(w), signifying the occurrence of lactose crystallisation as confirmed by both XRD analysis and glass transition temperature (T-g) analysis. The particle size (D-90) decreased for all the powders due to distortion of particle structure. Caking was observed in all samples conditioned between 0.330 and 0.753 a(w). The caking strength of P2 was significantly higher as compared to the other powders at 0.529 a(w), which suggests that magnesium content played a significant role in the hygroscopicity of the powders. These results demonstrate that small compositional differences have a profound effect on the moisture sorption behaviour and caking strength of the infant formulas. (C) 2015 Elsevier Ltd. All rights reserved.