We found that ibuprofen (IPF) became amorphous when the crystalline powder was merely mixed with polyvinylpyrrolidone (PVP), using a test tube mixer, and allowed to stand at appropriate temperatures. The crystallinity of the mixture was evaluated from the X-ray diffraction spectra by Ruland's method. The degree of decrease in crystallinity (A), defined as the ratio of the decrease in crystallinity to the value calculated by assuming additivity of crystallinity, increased gradually with time for the IPF-PVP system. We found that the higher the storage temperature, the higher the weight ratio of PVP in the mixture and the lower the molecular weight of PVP, the greater was the degree of decrease in crystallinity. The effect of PVP particle size was observed in 1 : 1 and 1 : 3 IPF/PVP mixtures.The interaction of IPF with PVP in the solid state was investigated by IR and ¹³C-NMR analyses, and the molecular geometry of 1-ethyl-2-pyrrolidone (EtP, a model compound for PVP) was calculated by the molecular orbital method (PM3). Our findings suggested that EtP undergoes O-protonation, forming a hydrogen bond at the carbonyl oxygen, in preference to N-protonation.