This work examines the effect of the particle size of Durum wheat semolina on the water diffusion coefficient and the evolution of the microstructure during hydration. Durum wheat semolina was sieved in 5 size categories. Water-particle interaction of powders (5 fractions and raw semolina) was studied using dynamic vapour sorption. Water sorption isotherms were fitted to G.A.B. and Y&N models. Monolayer values as well as multilayer properties were calculated and compared. Absorption properties were determined by Y&N model. A sorption curve was modelled with the Fickian diffusion equation adapted to water diffusion into solid particles. A dynamic Fickian water diffusion coefficient was calculated for each particle size after conditioning at different RHs. The surface structure of conditioned fractions was observed by ESEM. The microstructure of semolina presented an evolution with the increase of size and RH. Starch grains presented an increased size and protein matrix had a different texture. After correlation of isotherm data and microstructure, the resulting state and properties of the components were a key to understand diffusion on food powder systems.