IR and UV-vis spectroscopic measurements have been used to chart the thermal and photolytic reactions occurring in HCl-doped Ar matrixes containing Zn, Cd, or Hg atoms (M). Following the formation of a loosely bound metal atom adduct M...HCl, the primary reaction induced by UV irradiation (200 less than or equal to lambda less than or equal to 400 nm) is insertion of M into the H-Cl bond to give the molecule HMCl, which has been characterized by its IR spectrum, with assignments validated by the effects of isotopic change (H-1,H-2, Cl-35,Cl-37, and Zn-64,Zn-66,Zn-68) and by the results of density functional theory calculations. The properties of HMCl are compared with those of other Group 12 metal hydrides, and the mechanism of insertion is also discussed.