The aim of the present study was to examine the structural changes of amorphous polyvinylpirrolidone, associated with the physical ageing, by the determination of the free volume of the polymer and to find a correlation between the structural changes of the polymer and the physical stability of tablets containing it. The water absorption from the storage medium resulted in the glassy to rubbery transition of amorphous polyvinylpyrrolidone at room temperature. This spontaneous transition was not continuous in the case of PVP K25. Not only the actual water content but the storage history determines the free volume distribution of the samples. The structural changes associated with the physical ageing influence the physical stability of solid dosage forms containing this polymer.