We study the thermodynamics and chain dimensions of a highly interacting polymer mixture, poly(tetramethyl bisphenol A polycarbonate) and deuterated polystyrene (TMPC/PSd), using small angle neutron scattering (SANS). The one-phase region of the phase diagram is investigated as a function of blend composition phi and temperature T, employing three SANS diffractometers and covering a wide scattering wavevector window (0.008 < q < 0.25 angstrom(-1)). We find that the random phase approximation (RPA), accounting for component polydispersity, describes remarkably well all scattering data. Previous SANS measurements suggested anomalous mean-field behavior of TMPC/PSd, associated with a spatially dependent or "structured" Flory interaction parameter chi(q). We show that structure factors from Mixtures undergoing temperature jumps or quenched below the glass transition exhibit the same apparent deviations from RPA, Suggesting that incomplete equilibration could account for the observations. After demonstrating the regular RPA behavior of TMPC/PSd, the only miscible polystyrene and polycarbonate pair, we determine for the first time comprehensive mixture interaction parameters chi(phi, T) and the chain segment length for TMPC.