Positive mixing volume was detected by PVT measurements in immiscible but compatible blends PA/P(n-BA-co-MMA). An occurrence of additional free volume at phase boundaries is usually given as an explanation of the positive volume of mixing in case of blends of the immiscible polymers. A result of looking for the above-additive free volume above the glass transition temperature is given here. Results of Fourier Transform Infrared Spectroscopy support the idea that polyamide reacted (to some extent at phase boundaries) with the acrylic rubber, forming a graft or block copolymer, responsible for the improvement of compatibility of the constituents of the blends.