Isothermal crystallization of poly(butylene terephthalate) (PBT) blended with oligomeric poly(epsilon-caprolactone) (PCL) is investigated by polarized optical microscopy and differential scanning calorimetry at various temperatures (T-c). The growth rate of PBT spherulites is found to depend on time (t), as the spherulite radius (r) linearly increases with t at the early stages of crystallization (r proportional to t), then, with the progress of phase transition, the spherulite radius becomes dependent on the square root of the time (r proportional to t(1/2)) until termination of crystal growth. The nonlinear advance of the crystal growth front is caused by a varied composition of the melt phase in contact with the growing crystals, due to diffusion of mobile PCL chains away from the spherulite surface. The melt phase becomes spatially inhomogeneous, causing self deceleration of PBT crystallization until a limit composition that prevents further crystallization is reached in the melt. The maximum crystallinity achievable during isothermal crystallization decreases with T-c. The lowering of the temperature after termination of the isothermal crystallization allows to complete the crystal growth, but the final developed crystallinity still depends on T-c, being lower at higher T(c)s. (c) 2007 Wiley Periodicals, Inc.