The effects of diffusion of crystallizable polymer to a developing spherulite embryo are treated. Using the classical nucleation theory result for the size of a critical-size embryo and an existing model for the growth of a small pure component sphere in a bicomponent matrix, the time TB for a critical-size embryo to develop under diffusion control is given. The nucleation rate is lowered inversely proportionally to tip. The dependence of tau(D) on crystallization temperature exhibits a minimum. At temperatures above that minimum, tau(D) for a blend becomes increasingly larger with temperature than the time to form a nucleus in a pure system with similar diffusivity. Below the minimum, tau(D) decreases with increasing crystallization temperature. Decreasing the activation energy for diffusion causes the minimum to move to higher temperatures. (C) 2016 Elsevier Ltd. All rights reserved.