Amorphous solid solutions were obtained in different proportions by dissolving ketoprofen (Ket) and poly(vinylpyrrolidone) (PVP) K30 in methanol and by evaporating them under reduced pressure. Their physical properties were Studied by powder X-ray diffraction and by differential scanning calorimetry and compared to that of same physical mixtures (PM). The interaction of Keto and poly(vinylpyrrolidone) K30 in the solid state was then investigated by C-13 NMR CPMAS analysis. The results indicated that in the Ket-(PVP) co-precipitates, the ket molecules, interacting with their carboxylic group through hydrogen bonding with the poly(vinylpyrrolidone) moicties, are molecularly and irregularly dispersed within the amorphous sovendum that acts as mechanical substratum for amorphous stabilization. In their simple PM, no evidence exists of the Ket-polymer interaction during the considered time interval. (C) 2004 Elsevier B.V. All rights reserved.