We have measured the Flory-Huggins interaction parameter (chi) for blends of polystyrene (PS) and poly(4-vinylpyridine) (P4VP) by three different methods. First, we measured the micelle spacings in microphase-separated films of PS-P4VP diblock copolymers by secondary ion mass spectrometry and atomic force microscopy. Second, we measured the contact angle of droplets of homopolymer PS on P4VP homopolymer film. Finally we determined the interfacial width between homopolymer layers of dPS and P4VP by neutron reflectometry. From each of these experiments chi was calculated using mean field theory in the strong segregation limit. These values of chi are much larger than those of other nonionic polymer pairs. Pie discuss the importance of this findings.