Phase diagrams (cloud point curves, critical points, tie lines for constant critical composition) and interfacial tensions as a function of temperature were measured for solutions of two random copolymers: poly(dimethylsiloxane-ran-methylphenylsiloxane) [I] and poly(sty rene-ran-acrylonitrile) [II]. Acetone and anisole served as solvents for I and toluene for II; all solutions exhibit UCSTs between 300 and 310 K. The phase separation behavior can be well modeled if one accounts for the molecular and chemical non-uniformities of the random copolymers used in this study. The interfacial tensions sigma differ most markedly from that of comparable homopolymer solutions in their correlation sigma = sigma(tau)tau(mu), where tau = (T-c - T)/T-c. For all three systems sigma(tau) results considerably less and the critical exponent mu varies widely from 0.68 to 2.18 (in contrast to the normal case where mu is on the order of 1.3-1.5). Both observations are explained in terms of the capability of copolymers to minimize the interfacial energy by suitably arranging their different monomeric units. Model calculations were performed in terms of the energy required to transfer molecules from one phase to the other, assembling the average polymer-solvent interaction parameter from the three binary interaction parameters g(ij), required to describe copolymer solutions. These results demonstrate that the experimentally observed particularities of copolymers are more Likely dominated by dissimilarities in the concentration dependence of g(ij) than by unlike temperature dependencies. Particularities in the correlations of the length of the tie line with tau and sigma, respectively, are also discussed. (C) 2000 Elsevier Science Ltd. All rights reserved.