The chemical interaction between Al-Si melts of different compositions and graphite was investigated in order to clarify the mechanism of spontaneous growth of silicon carbide crystals from these melts. Calibrated graphite small rods were used as carbon source to facilitate comparison between experiments. For a temperature set to 1100degreesC, the reaction time and Si content of the melt were varied from 1 to 48 hours and 20 to 40 at.%, respectively. It has been found that in a first stage the liquid reacts at a relatively slow rate to form a microcrystalline SiC layer around the graphite rod. When this SiC layer has reached a certain thickness, a violent attack follows in some specific sites by rapid dissolution of the rod. Radial liquid channels progress from the surface of the rod up to its centre and then total conversion of graphite into SiC rapidly occurs. The local Si content of the melt, which controls the carbon solubility in the liquid, governs the overall mechanism. To form faceted beta-SiC crystals, the growth mechanism should involve carbon dissolution in one place and supersaturation in another place in relation with local changes of the Si content in the melt.