[en] The simplest carbon compounds, present in the terrestrial and planetary atmospheres, exhibit a wide range of oxidation states, carbon dioxide and methane being the most oxidized and the most reduced form of carbon, respectively. The question arises as to the origin and the interconversion among the carbon species. The chemical pathways for the conversion of CH₄ to CO and CO₂ are for the most part known. The reverse process, the reduction of CO to CH₄ is however, poorly understood. A new reaction is proposed, H₂CO + H + M yields CH3O + M, which might play a fundamental role in the reduction of CO or CH₄. An update is presented of nitrile photochemistry on Titan