The physical aging of poly(vinylpyrrolidone) (PVP K25) was studied under different storage conditions by positron lifetime spectroscopy and scanning electron microscopy. The transition from the glassy state (at RT and 55% RH) to the completely plasticized wet rubbery state (at RT and 75% RH) is not continuous in PVP K25. It was found that not only the actual water content of the material but also its storage history determines the size distribution of free volume holes in it. At 65% RH a slow anomalous structure formation was observed. By the means of ab initio calculations on simplified model molecules, it was determined that polymer chains and water molecules are able to form a hydrogen bound "network" under certain humidity conditions.