We have determined the segregation of diblock copolymers of deuterated polystyrene (dPS) and poly(2-vinylpyridine) (PVP) to a planar interface between the homopolymers using forward recoil spectrometry. The segregation isotherms, namely the interfacial excess versus its volume fraction in the homopolymer phase far from the interface, for the diblock copolymers were measured at several temperatures. Using a self-consistent mean field (SCMF) theory, one can fit the segregation data accurately if the Flory interaction parameter chi is treated as a fitting parameter. Using this scheme, the temperature-dependent Flory interaction parameter chi between PS and PVP was determined to be -0.033+63/T(K). In contrast to this evaluation of chi which depends heavily on a model, we have shown that the isosteric heat of segregation q(seg) can be directly extracted from the experimental data. We demonstrate that the enthalpic portion of the Flory interaction parameter chi(H) can be estimated directly from q(seg), without recourse to a model of the segregation layer, and the chi(H) from this direct method is in good agreement with that determined using the SCMF theory. These two methods provide a new approach for determining chi for strongly immiscible polymers.