The title radicals were produced by femtosecond collisional electron transfer in the gas phase and studied by the methods of variable-time neutralization-reionization mass spectrometry combined with fast-beam laser photoexcitation and G2(MP2) ab initio/RRKM calculations. The methylsulfonyl radical (CH3SO2., 1) was calculated to be bound by 59 kJ mol(-1) against the lowest-energy dissociation to CH3. and SO2 at 0 K and to have a heat of formation Delta H-f,H-298(1) = -211 +/- 4 kJ mol(-1). When formed by vertical electron transfer, radical 1 dissociated rapidly due to a large Franck-Condon energy, E-FC = 141 kJ mol(-1). The reverse addition of CH3. to the sulfur atom in SO2 had a potential energy barrier of 1.3 kJ mol(-1) and Arrhenius parameters, log A = 12.19 and E-a = 5.4 kJ mol(-1). The calculated addition rate constant, k(295) = 1.7 x 10(11) cm(3) mol(-1) s(-1), was in excellent agreement with the previous measurement of Simons et al. The methoxysulfinyl radical (CH3OSO., 2) was calculated to exist as an equilibrium mixture of syn (2s) and anti (2a) conformers. The Boltzmann-averaged heat of formation of 2 was calculated as Delta H-f,H-298(2) = -230 +/- 4 kJ mol(-1). Vertical neutralization of ions 2s(+) and 2a(+) produced substantial fractions of stable 2s,a. Dissociating 2s,a formed CH3. and SO2 through unimolecular isomerization to 1. Direct dissociation of the C-O bond in 2s,a to form CH3. and SO2 was calculated to have a large activation barrier (152 kJ mol(-1) from 2a) and did not compete with the isomerization to 1, which required 111 kJ mol(-1) from 2a. Photoexcitation of 2s,a resulted in a slightly increased formation of 2s,a(+). This was interpreted with the help of CIS/6-311+G(3df,2p) calculations as being due to the formation of a bound excited B state of 2s upon electron transfer. The B state was photoexcited at 488 and 514.5 nm to high Rydberg states which were predicted to have large cross sections for collisional ionization. The A state of 2s was calculated to be bound but photoinactive. The C through E states of 2s were unbound and predicted to dissociate exothermically to CH3OS and (P-3)O.