Abstract Methane is typically thought to be formed in the solid state on top of cold interstellar icy grain mantles via the successive atomic hydrogenation of a carbon atom. In the current work we investigate the role of molecular hydrogen in the CH 4 reaction network. We make use of an ultrahigh vacuum cryogenic setup combining an atomic carbon atom beam with atomic and/or molecular beams of hydrogen and deuterium on a water ice. These experiments lead to the formation of methane isotopologues detected in situ through reflection absorption infrared spectroscopy. Most notably, CH 4 is experimentally formed by combining C atoms with only H 2 on amorphous solid water, albeit more slowly than in experiments where H atoms are also present. Furthermore, CH 2 D 2 is detected in an experiment involving C atoms with H 2 and D 2 on H 2 O ice. CD 4 , however, is only formed when D atoms are present in the experiment. These findings have been rationalized by means of computational and theoretical chemical insights. This leads to the following conclusions: (a) the reaction C + H 2 → CH 2 takes place, although it is not barrierless for all binding sites on water, (b) the reaction CH + H 2 → CH 3 is barrierless, but has not yet been included in astrochemical models, (c) the reactions CH 2 + H 2 → CH 3 + H and CH 3 + H 2 → CH 4 + H can take place only via a tunneling mechanism, and (d) molecular hydrogen possibly plays a more important role in the solid-state formation of methane than assumed so far.