Abstract Most experimental methods for following the changes in crystallinity of polymeric systems have involved measurement of volume changes, changes in absorption spectra, and changes in optical properties. The basis of the novel method reported here is that the bulk interaction between small concentrations of a “probe” molecule and a polymer substrate depends in a simple manner on the degree of crystallinity of the polymer sample. In a recent paper from this laboratory it was suggested that this bulk interaction may be measured by a gas chromatographic method. In this paper, the method is extended to measuring the rate of polymer crystallization; a linear and a branched chain polyethylene) were studied using decane as the “molecular probe”.