Molecular interactions in miscible polymer blends influence glass transition (T-g) behaviour and the increment of heat capacity at T-g. Whether the glass transition temperature and the increment of heat capacity versus composition show positive or negative deviations from linearity for blend systems depends on specific molecular interactions. In the non-interacting system, poly(vinyl acetate)/poly(methyl acrylate), the glass transition temperature and the increment of heat capacity show a linear behaviour with composition. Thermodynamic theoretical considerations concerning the non-linear behaviour of the glass transition temperature and the increment of heat capacity with composition in miscible polymer blends are developed in this paper.