A previous theory of thickening has been extended to encompass the possible dependence of cooperative chain transport on lamella thickness and crystallinity changes during annealing. These have marked effects on the thickening rate leading to changes in slope and departures from straight‐line behavior of the plots of lamella thickness versus the logarithm of time. The resulting curves bear a resemblance to many exprimental ones in the literature. Also brought out by the analysis is the important fact that the volumes of the coherently thickening domains are not known from experiment.