Blends containing poly(N-1-alkylitaconamic acids) (PNAIA) and poly(N-vinyl-2-pyrrolidone) (PVP) of two different weight average molecular weights, were studied by differential scanning calorimetry (DSC), Thermogravimetric analysis (TGA), and infrared spectroscopy (FTIR). The phase behaviour of the blends is analyzed in terms of the side chain structure and the specific interactions involved, mainly due to the free carboxylic group and the amide groups in PNAIA and the carbonyl group of PVP. The strength of the interaction was analyzed in terms of the Gordon-Taylor parameters.