A simple model is proposed to derive the Gibbs potential change associated with the formation of secondary nuclei in the crystallization of systems consisting of isotactic polypropylene nucleated with small amounts of indigo. A basic assumption is that the average basal surface tension of critical nuclei lowers as an effect of chain–nucleant interaction. The actual fold length of individual stem pairs may be lower than critical due to chain–nucleant encounters. Two limiting cases are considered, namely, when the fold lengths of adjacent stem pairs are independent of one another (random fold), and when they are not (persistence). Corresponding expressions of the nucleation free enthalpy are used to analyze available data of crystallization kinetics. Both scenarios describe the physical situation well, most likely because under the actual experimental conditions critical nuclei consist, on average, of one stem pair. The random fold scheme, however, is more informative as it provides (at least in principle) a method for determining both the basal and lateral contributions to surface tension through an analysis of the crystallization kinetics. ©1997 American Institute of Physics.