Flame instabilities and the formation of cellular structures during spherical gaseous explosions have been studied experimentally using natural light and schlieren high-speed cine photography, as well as single-shot planar laser-induced fluorescence (PLIF) from the OH radical. High-pressure, rich-hydrocarbon and lean-hydrogen flames at low Markstein numbers were employed. Ranges of unstable wavelengths have been identified as a function of Markstein and Peclet numbers. The cine photography enables the dynamics of cell growth and fissioning to be studied and qualitatively interpreted, in terms of flame stretch rates and thermodiffusion. The PLIF technique enabled unstable wavelengths to be measured and flame fracture at negatively stretched cracks to be observed. A cascade of unstable wavelengths terminates in a cellular structure. This structure appears at a second critical Peclet number. The smaller cells are continually destabilizing and restabilizing. As they increase in size, the localized stretch rate on the cell surface decreases and the cell becomes unstable. It restabilizes by fissioning into smaller cells with higher localized stretch rates. The cells are bounded by cracks in regions of negative curvature. At sufficiently small Markstein numbers the cracks are fractured. The results are interpreted within the theoretical framework of the stability analysis of Bechtold and Matalon. (C) 2000 by The Combustion Institute.