In the present work, the carbonization of porous silicon for the subsequent 3C-SiC growth has been systematically studied. The effect of temperature and acetylene flow-rate on the chemical state of the surface and structure relaxation was studied. It was found that the porous nano-crystalline morphology is unstable and tends to recrystallize in temperature range typical of 3C-SiC growth on Si (10000C-13000C). The carbonization impedes recrystallization at 10000C, but at 13000C the full recrystallization takes place. Pyrolytic amorphous graphite-like carbon was found on porous silicon carbonized at temperature and with acetylene flow-rate above critical values.