The order-to-disorder transition (ODT) of 21 compositionally symmetric diblock copolymers consisting of various combinations of poly(ethylene) (PE), poly(ethyl ethylene) (PEE), poly(ethylene-propylene) (PEP), poly(ethylene oxide) (PEO), and poly(dimethylsiloxane) (PDMS) was used as a measure of the strength of the interaction between the polymers. Conformational asymmetry makes a contribution to polymer-polymer thermodynamics that is not explainable within existing simple theories. For a series of PDMS-hydrocarbon diblock copolymers the observed phase behavior can be rationalized through a self-consistent set of solubility parameters. However, this model as well as other models breaks down when the phase behavior of a series of PEO-hydrocarbon block copolymers is included. We suggest that conformational. asymmetry leads to an enthalpic contribution to the Flory-Huggins parameter chi through an effective coordination number which is maximized when both of the blocks have large conformational. symmetry parameters, beta(2).