Optical, electron metallography and X-ray diffraction techniques were employed to characterize the microstructure of a rapidly solidified Al-12 wt % Si alloy. Ribbons produced with the planar flow casting process at a cooling rate of 10(6) Ks(-1), had a very fine cellular structure of alpha Al and nanosize Si particles which are distributed inside and at the boundaries of these cells. Thin ner sections (t < 30 mu m) have apparently experienced higher cooling rates and show, in addition to a cellular structure, a distinct zone which is featureless at optical microscope magnifications.