The influence of impurity segregation on the kinetics of spherulitic crystallization is examined. Radial growth rates over a range of temperatures are reported for a number of polymers containing various proportions of impurity species of widely varying molecular weight. Variations of growth rate with molecular weight indicate that transport processes within regions extending some distance from solid‐liquid interfaces play a major part in controlling growth kinetics. In the unusual case of a polymer containing impurities of very low molecular weight, extended radial diffusion of impurities may even give rise to parabolic growth [(radius) ∝ (time)½] in place of linear growth as normally found. Conventional interpretations fail to account satisfactorily for dependence on molecular weight, and are clearly inadequate in that they deal with transport processes only as ``jump'' processesatthese interfaces. Complications that transport processes within the body of the melt introduce into the problem of formulating an absolute theory of growth kinetics are discussed.