We report the infrared absorption spectrum of the methylthio (or thiomethoxy) radical, CH3 S (X E2 3/2), produced via photodissociation in situ of three precursors CH3 SH, CH3 SCH3, and CH3 SSCH3 isolated in solid p -H 2. The common absorption features observed with similar intensity ratios in each experiment are assigned to CH3 S. The wavenumbers of these features agree satisfactorily with those predicted with a spin-vibronic Hamiltonian accounting for the anharmonic effects and the Jahn-Teller effects to the quartic term [A. V. Marenich and J. E. Boggs, J. Chem. Theory Comput. 1, 1162 (2005)]. In addition to an absorption line at 724.2 cm-1, corresponding to a transition of 31 previously determined to be 727 cm -1 from fluorescence spectra of gaseous CH3 S, we identified fundamental transitions 61 (a1) at 771.1, 61 (e) at 1056.6, 51 (a1) at 1400.0, 41 (a1) at 2898.4 cm -1, and several combination and overtone transitions. In contrast, photolysis of CH3 SSCH3 isolated in solid Ar produces mainly H2 CS, CH3 SH, and CS2, but no CH 3 S. These results demonstrate the feasibility of using photolysis in situ of precursors isolated in solid p -H2 to produce free radicals by taking advantage of the diminished cage effect of the matrix.