Thin films of polymers (polysiloxanes, polycarbosilanes, and polysilazanes) and alkoxide-derived siloxane gels, precursors for SIG, SIGN, SiOC, and SiOBC ceramics, were irradiated with increasing fluences of C or An ions to study the kinetics of their conversion into ceramics. Ion beam analyses showed that the main effect of irradiation on the composition of the films is the selective release of H-2 by radiolysis, During subsequent high-temperature annealing of films converted as much as possible by irradiation, COx, CHx, or silane molecules do not evolve, contrary to what is observed during the pyrolysis of unirradiated precursor films. According to Raman analyses, a large proportion of the carbon atoms segregate into clusters after irradiation and in films converted by direct pyrolysis (or combined treatments). However, carbon particles formed during irradiation are more diamond-like, affording films with 2-3 times higher hardness, as shown by nanoindentation tests, In both types of ceramics (SiC or SiOC), the optimal properties (hardness, thermal stability, and photoluminescence) associated with C segregation are obtained for a C/Si ratio of the order of 1, Boron addition is detrimental to hardening of SiOC glasses, in contrast to nitrogen.