The kinetics of domain size equilibration were studied for asymmetric poly(ethylene-alt-propylene)-b-poly(dimethyl siloxane) (EPDMS) and polyisoprene-bpoly(dimethyl siloxane) (IDMS) block copolymers in the body-centered cubic ordered phase. Small-angle X-ray scattering measurements of the principal peak position (q*) were made as a function of time after temperature jumps within the ordered state. The equilibration times were remarkably long, especially on cooling and for temperatures below 100 degreesC. For example, after a quench to 40 degreesC, q* for EPDMS had not fully equilibrated even after several weeks of annealing; IDMS required several days to equilibrate at the same temperature. In contrast, a lamella-forming EPDMS sample was able to adjust q* within the timescale of the measurements (i.e., minutes) with both heating and cooling over the same temperature range. Measurements of tracer diffusion indicated that chain mobility was not the rate-limiting step, although differences in mobility did account for the differences between EPDMS and IDMS. Rather, the limiting step was the required reduction in the number density of spheres on cooling; the disappearance of spheres, either by evaporation or by fusion, provided a large kinetic barrier. Lamellae, however, could adjust domain dimensions simply by local displacements of individual chains. (C) 2003 Wiley Periodicals, Inc.