Phase diagrams in the vicinity of liquid-liquid demixing for polystyrene/solvent systems have been determined as functions of polymer molecular weight, polydispersity, pressure, temperature, solvent quality, and isotope substitution. The diagrams show upper and lower consolute branches and associated critical and hypercritical loci. In this paper we present preliminary results from some studies of demixing along a near-critical isopleth using laser light scattering and photon correlation techniques (dynamic light scattering, DLS), and small angle neutron scattering (SANS). (C) 1998 Elsevier Science B.V. All rights reserved.