A microcellular ceramic with cell densities >10(9) cells/cm(3) and cells <10 mum was made with a preceramic mixture of polycarbosilane and polysiloxane. The preceramic compact was saturated with gaseous CO2, a large number of cells were nucleated and grown by using a thermodynamic instability induced by a rapid pressure drop, and the microcellular preceramic was transformed into a microcellular ceramic by pyrolysis.