Industrially, after the crystallisation of lactose from mother liquor, the crystals are separated by centrifugation and then dried in a flash drier followed by a fluid bed drier. It is known that if the moisture content and/or particle size is not correct then lumping and caking of the lactose can occur in the flash drier causing operational problems.The aim of this study was to characterize the influence of water content and particle size distribution on the caking of lactose powders. Powders with different d50's and different water contents had their caking/clumping characteristics analyzed with a texture analyzer, a sticky plate and a blow tester to determine their cohesion. A theoretical approach looking at the capillary interactions of the liquid bridges between lactose crystals was taken from the literature to explain the results obtained. The results showed that lactose powder with a low d50 can cake easily as soon as the water content is higher than 3%. This cohesion between lactose crystals is due to liquid bridges linking them together. The number of bridges formed affects the level of cohesion: the more numerous they are, the more cohesive the lactose crystals are. Therefore, small particles cake more easily because the number of bridges in a given volume is higher than for bigger particles. It was predicted that the capillary force cannot hold together particles with d50 bigger than 400 μm and that was confirmed by experiment. Finally, in order to prevent the caking of lactose powders obtained by processes such as a decanter centrifuge, the lactose crystallization step should be controlled to obtain large crystal (with d50>300 μm) and/or the final water content should not be higher than 3%.