We report new measurements of the liquid−liquid phase separation curves of poly(α-methylstyrene) + methylcyclohexane, for three well-characterized samples of the polymer with number average molecular weights of (A) 19 700, (B) 32 300, and (C) 72 000 g/mol. Sample A (polydispersity index PI = 1.02) has a critical point at a mass fraction m c of polymer of 0.20 ± 0.02 and a critical temperature T c at (305.70 ± 0.09) K. The critical point for sample A is not at the apex of the phase separation curve, which indicates that the polymer was not sufficiently monodisperse to behave as one component. Sample B (PI = 1.02) has m c = 0.15 ± 0.02 and T c = (318.3 ± 0.4) K; this critical point is at the apex of the phase separation curve within our resolution. Sample C (PI = 1.05) has m c = 0.11 ± 0.02 (not the apex of the curve) and T c = (328.9 ± 0.4) K. From these measurements, we find a theta temperature ϑ of (357 ± 5) K and an entropy of dilution parameter ψ1 of 0.49 ± 0.06. The polymer/solvent interaction parameter χ1 at T c for each sample is (A) 0.54 ± 0.01, (B) 0.56 ± 0.01, and (C) 0.58 ± 0.01. The uncertainties for ϑ, ψ1, and χ1 are given as one standard deviation.