The flow properties of spray-dried solids depend on their composition and physical characteristics. This study investigated the influence of water plasticization and lactose content on flow properties of lactose/milk protein isolate (MPI) solids systems. Median particle size (d(50)) of lactose/MPI solids systems increased as lactose content decreased, and lactose showed the largest specific surface area (SSA). Particle shape of dairy solids with higher lactose content had less rounded shape, rougher surface and lower ratio of width/length. T-g values of lactose/MPI mixtures (protein content <= 60%) showed only minor difference as comparing to T-g values of pure lactose in this study. Mechanical study showed that the higher was the lactose content in dairy solids, the more significant was the change in their modulus at glass transition region. Lactose/MPI mixtures with higher lactose contents showed better flowability in this study, but they gave bigger friction angles after storage at same relative humidity. (C) 2015 Elsevier Ltd. All rights reserved.