IR and UV/vis spectroscopic measurements have been used to chart the reactions set in train by irradiating methane-doped argon matrices containing gallium or indium atoms, first with light having λ ) 200-400 nm, and subsequently with broad-band UV-visible light. Assignments of the IR absorption bands are made and their carriers characterized on the basis of D and 13 C substitution and by comparison either with the vibrational properties of related molecules or with those forecast by DFT calculations. Photoexcitation at λ ) 200-400 nm results in insertion of the group 13 metal atom into a C-H bond to form the angular monomethylmetal hydride molecule, CH 3 MH (M ) Ga, In). This product is not photostable but decomposes on exposure to radiation with λ ) 200-800 nm to give the first sighting of CH 3 M, the simplest organic derivative of Ga(I) and In(I) to be characterized to date. The vibrational properties of the species CH 3 MH and CH 3 M are compared with those of related molecules and analyzed for the light they shed on the characters of the M-C and M-H bonds.