The coexistence curves of blends of polystyrene (PS, M(w) = 600 and 2.2 x 10(3) g mol(-1)) and poly(dimethylsiloxane) (PDMS, M(w) = 460, 1400, 3700 and 2.3 x 10(5) g mol(-1)) were determined using a specially designed refractometer. The coexistence curve of the oligomer blend PS (M(w) = 600)/PDMS (M(w) = 460) was of the upper critical solution temperature type, and sensitively reflected molecular-weight polydispersity of the PS, although the polydispersity index M(w)/M(n) was less than 1.10. The coexistence curve at the critical composition could be fitted to phi(+/-) = a(+/-)epsilon(beta) + b(+/-) epsilon(beta+Delta), where phi(+) and phi(-) are the volume fractions of PDMS of the coexisting phases (phi(+)> phi(-)), the exponent Delta is 0.5 and E is the reduced temperature. The critical exponent beta was determined to be 0.33(6), which was that for non-renormalized Ising behaviour, showing no polydispersity effect. The Flory-Huggins theory could reasonably describe the coexistence curves of blends with different molecular weights, although it showed a slight discrepancy for the effects of molecular-weight asymmetry on the coexistence curve. By fitting the calculated coexistence curves to the experiments, the interaction parameter chi was evaluated as a function of temperature. The obtained chi parameter involved a large enthalpy term, dominating the entropy term, and indicated that PS/PDMS was an extremely immiscible blend compared with other reported blends. Critical fluctuation effects on the coexistence curve were also discussed.