A new molecular weight/polarity map was developed for petroleum residua based on the Scatchard-Hildebrand solubility equation. A series of extractions is performed with solvents of increasing solubility parameter, and the fractions are analyzed by vapor pressure osmometry for number average molecular weight, and analytical scale size exclusion chromatography for molecular weight spread. The results are diagnostic of the layers of solvation by resin-type molecules around a central asphaltene core. As thermal treatment progresses, the solubility parameter of the components of residua fractions increases, and the apparent molecular weight decreases. Coking onset appears to coincide with the depletion of resin-type asphaltene solubilizing components of residua and the formation of a bimodal system. (C) 2001 Elsevier Science Ltd. All rights reserved.