The modes of formation of carbonaceous deposits ("coke") during the transformation of organic compounds over acid and over bifunctional noble metal-acid catalysts are described. At low reaction temperatures, (< 200 degreesC) "coke" formation involves mainly condensation and rearrangement steps. Therefore, the deposits are not polyaromatic and their composition depends very much on the reactant. The retention of the "coke" molecules on the catalysts is mainly due to their strong adsorption and to their low volatility (gas-phase reactions) or to their low solubility (liquid-phase reactions). At high temperatures (> 350 degreesC), the coke components are polyaromatic. Their formation involves hydrogen transfer (acid catalysts) and dehydrogenation (bifunctional catalysts) steps in addition to condensation and rearrangement steps. On microporous catalysts, the retention of coke molecules is due to their steric blockage within the micropores.