The deposition of carbon on catalysts during the partial oxidation of methane to synthesis gas has been investigated and it has been found that the relative rate of carbon deposition follows the order Ni > Pd > Rh > Ir. Methane decomposition was found to be the principal route for carbon formation over a supported nickel catalyst, and electron micrographs showed that both "whisker" and "encapsulate" forms of carbon are present on the catalyst. Negligible carbon deposition occurred on iridium catalysts, even after 200 h.