Poly(methyl methacrylate) (PMMA) and poly(alpha-methylstyrene-co-acrylonitrile) (P alpha MSAN) form partially miscible blends with lower critical solution temperature (LCST) behavior. We revisit this system using small angle neutron scattering (SANS), examining the effect of molecular weight (M-w) of deuterated PMMA (dPMMA), blend composition (phi), and temperature (T) in the homogeneous region. All data are well-described by the Random Phase Approximation (RPA) theory, enabling us to determine thermodynamic and structural parameters, including the correlation length xi, G '' (the second derivative of the free energy of mixing with respect to composition), and the statistical segment length a of each component. Phase boundaries are computed by extrapolation of G '' with temperature, to yield the spinodal, and inspection of Kratky plots to yield the binodal. For P alpha MSAN, a is determined to be 10.1 +/- 0.4 angstrom. Unsurprisingly, this system deviates strongly from Flory-Huggins expectations, exhibiting a minimal M-w, dependence of the phase boundaries and phi dependence of effective interaction parameter ((chi) over tilde). Comparison of G '' with values for other blend systems places P alpha MSAN/dPMMA in a class of highly interacting blends, expected from Cahn-Hilliard theory to yield small initial phase sizes upon spinodal demixing. This is confirmed experimentally, with an illustrative temperature jump resulting in an initial phase size of similar or equal to 30 nm.