The physical properties of four commercial dental composites were investigated through differential scanning calorimetry, water sorption and desorption measurements and flexural mechanical properties tests. The differential scanning calorimetry curves of samples as prepared and after different times of ageing in water indicated that the small residual monomer reactivity, present in the as prepared samples, disappeared after immersion in water, which probably acts as a plasticizer and facilitates a further crosslinking reaction of the material and the residual monomer desorption. Consistently, water causes the embrittlement of the material, as detected from the flexural mechanical properties. Water sorption and desorption kinetics were measured at different temperatures, the water diffusion coefficients were calculated and the activation energies of the diffusion process were determined. The SEM analysis of the fracture surfaces and the decrease of the water uptake on the temperature indicated the existence of a good filler/matrix adhesion.