In the present Monte-Carlo simulation, we study the effects of structural differences on the thermodynamic behavior of polymer blends within the framework of the bond fluctuation model. Two types of structural differences are investigated : nonadditivity induced by indented monomer shapes and different stiffnesses of the two components. In both cases we find evidence for a positive, chain length independent entropic contribution to the chi parameter. Within the semi-grand-canonical ensemble this contribution can be detected via corrections to the Flory-Huggins equation of state (SG-EQS) or a shift in the critical temperature. For mixtures with nonadditive monomers, we find an UCSP for repulsion between unlike species and short chain lengths and a LCSP for longer chains and attractive interactions. The simulation data are compared to a mean field approximation. The results of blends with different stiffnesses is in qualitative agreement with recent field theoretical calculations of Liu and Fredrickson.