Poly(methyl methacrylate) (PMMA) and poly(α-methyl styrene-co-acrylonitrile) (PαMSAN) form partially miscible blends with lower critical solution temperature (LCST) behaviour. We revisit this system using small angle neutron scattering (SANS), examining the effect of molecular weight (Mw) of deuterated PMMA (dPMMA), blend composition (φ) 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 ξ, G00 (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 G00 with temperature, to yield the spinodal, and inspection of Kratky plots to yield the binodal. For PαMSAN, a is determined to be 10.1±0.4 ˚A. Unsurprisingly, this system deviates strongly from Flory-Huggins expectations, exhibiting a minimal Mw dependence of the phase boundaries and φ-dependence of effective interaction parameter (˜χ). Comparison of G00 with values for other blend systems places PαMSAN/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 ' 30 nm.