Changes in the lamellar morphology that occurred during the quiescent isothermal crystallization of absorbable poly(p-dioxanone) (PDS) and PDS/poly(glycolide) block copolymer were studied by synchrotron small-angle X-ray scattering. Important morphological parameters such as the lamellar long period, the thicknesses of the crystal and amorphous phases, and the scattering invariant were estimated as a function of time, and trends observed over a nide range of experimental conditions are discussed. Thicker but more perfect lamellae were detected at higher crystallization temperatures. The breadth of the normalized semilog Lorentz-corrected intensity peak systematically decreased with increasing temperature. In addition, the values of the crystallization half-time and the Avrami exponent (n = 2.5), determined from the real-time changes in the lamellar development, showed superb agreement with the bulk crystallinity data generated from other experimental techniques, such as calorimetry and dielectric relaxation spectroscopy. (C) 2000 John Wiley & Sons, Inc.