The atmospherically highly relevant methylsulfonyl radical (CH3SO2 center dot) was generated by high-vacuum flash pyrolysis (HVFP) of allylmethylsulfone and isolated in an argon matrix at 10 K; the allyl radical formed as the cofragment. Upon thermolysis, the methylsulfonyl radical undergoes partial decomposition, leading to substantial amounts of sulfur dioxide in the matrix. The title compound was characterized through the assignment of eight fundamental IR bands of its CD3 and (CH3)-C-13 isotopologues and the excellent agreement with the B3LYP/6-311+G(3df,3pd) computed harmonic vibrational frequencies. The two most intense absorptions were found at 1267.1 and 1067.6 cm(-1). In extension of this study S-methyl methanethiosulfonate was found to be another suitable, although less efficient, precursor for the gas-phase generation of the methylsulfonyl radical.