The miscibility behaviour of blends of polystyrene and poly(p-methylstyrene) was studied by differential scanning calorimetry and phase-contrast optical microscopy. Phase diagrams have been constructed for blends of different molar masses of the constituents. These blends exhibit upper critical solution temperature type of demixing, which is a rare phenomenon for high-molar-mass polymer blends. The Holes-Huggins theory, closely related to the Simha-Somcynsky hole theory, predicts this type of phase behaviour for this particular system in a semi-quantitative way. Theoretical predictions are made for the influence of pressure on the miscibility behaviour and for the excess volume and enthalpy for this blend.