The isothermal crystallization of poly(l-lactide) (PLLA) in blends with poly(butylene oxalate) (PBOX) is investigated by time-resolved small-angle X-ray scattering, differential scanning calorimetry, and optical microscopy. We focus on the temperatures at which only PLLA crystallizes while PBOX is amorphous. It is obtained that the addition of PBOX causes a reduction of the melting temperature of PLLA. The lamellar thickness of PLLA crystals decreases whereas the amorphous layer thickness increases with blend composition, suggesting the occurrence of the interlamellar incorporation upon the addition of PBOX. The crystal growth rate and morphology of PLLA/PBOX blends are analyzed by polarized optical microscopy. The spherulite growth rate of PLLA is found to increase with the addition of PBOX. Analysis of the isothermal crystallization in terms of the Lauritzen and Hoffman equation give the reduction of the fold surface free energy upon the addition of PBOX in PLLA, indicating that the mobility of the PLLA chains is significantly improved due to the presence of PBOX. (c) 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 192-202