Sol-gel-derived SiOC ceramics are metastable glasses that undergo microstructural modifications upon annealing at temperatures exceeding 1000 degrees C. Typically, silicon-rich SiOC compounds show the presence of a low volume fraction of silicon and silicon carbide nanocrystals upon exposure to 1200 degrees C. Here, the microstructural changes occurring in a Si-rich SiOC composition after long-term annealing experiments performed at 1300 degrees C for 1, 10, and 100 h were investigated by high-resolution (HRTEM) and energy-filtered transmission electron microscopy (EFTEM), in order to characterize the size evolution of the Si precipitates. The elemental maps obtained through EFTEM analysis indicate that the nanocrystals grow as a function of the annealing time according to the classical mechanism of Ostwald ripening. The estimated diffusion constant of silicon in the SiOC matrix is on the order of 7 x 10(-17) cm(2)/s.