Blending (or mixing) of macromolecules is widely used to tailor the properties of polymeric materials and small-angle neutron scattering (SANS) has provided detailed information at the molecular level on the ability of different polymer species to mix or segregate at various thermodynamic conditions. For two decades, SANS data have been analyzed via the de Gennes "random phase approximation" (RPA) [P.-G. de Gennes, Scaling Concepts in Polymer Physics, second ed., Cornell University Press, Ithaca, London, 1979], which is based on the assumption that the dimensions of polymer chains remain unchanged on mixing for all concentrations and temperatures. Here we investigate the effect of temperature and concentration on the dimensions of macromolecules in blends using SANS and high-concentration labeling methods and construct a generic phase diagram, which specifies the range of validity of the RPA. Using scaling arguments, we demonstrate a parallel between the structure-property relationships in blends and solutions of polymers in small molecule solvents and reveal the impact of the chain length of the polymeric solvent on the phase behavior of polymer blends. The results offer new insights into the universality of the thermodynamic properties and structure of macromolecules in polymeric, liquid and supercritical solvents. (C) 2003 Elsevier B.V. All rights reserved.