The recent introduction of multiangle light‐scattering detectors for size‐exclusion chromatography has made possible the measurement of the root mean square radius of gyration (Rg) and molecular weight (M) of polymer fractions eluting from a size‐exclusion chromatography column. The characterization of the dimensions of a polymer may be accomplished with only a few milligrams. The dimensions of a polyimide precursor prepared by the condensation of the meta‐diethyl ester of pyromellitic dianhydride with para‐phenylene diamine have been measured with this technique. The dependence of Rg on M across the distribution is compared with that predicted for a freely rotating chain, and with other similar polymers measured with hydrodynamic techniques. Copyright © 1992 John Wiley & Sons, Inc.