It is shown that experimental Tg vs. composition data of miscible polymer blends can be adapted by a third order Tg vs. composition equation characterized by two parameters, K-1 and K-2, which result in the supposition that besides the thermodynamically claimed favourable energetic hetero-interaction, contributions of conformational mixing entropy are responsible for miscibility. K-1 comprises mainly energetic interaction effects, whereas K-2 is due to effects of the conformational mixing entropy. It further results surprisingly that K-1 is roughly correlated with the difference of the solubility parameters of the blend components. At the moment no explanation can be offered for this observation.