Rate constants for thermal electron attachment to SO2 in pure SO2 and SO2–M mixtures (M=N2, CO2, and n-C4H10) have been measured using the pulse radiolysis microwave cavity method in the pressure range from 0.4 to 700 Torr and at temperatures between 253 and 343 K. In all systems, the pressure dependence of the effective two-body attachment rate constants shows a combination of two saturation behaviors. The rate constants vary significantly with the nature of the medium gases, but very little temperature dependence has been observed. A new attachment mechanism has been proposed in which two different two-step, three-body processes (distinguished by I and II) occur simultaneously, and each involves different compound-negative-ion states of the SO−2 ion. The initial two-body attachment rate constants of (3.7±0.2)×10−12 and (5.0±0.5)×10−11 cm3 molecule−1 s−1 have been determined for processes I and II, respectively. Process II becomes dominant at higher gas pressures. The three-body rate constants range from 10−31–10−28 cm6 molecule−2 s−1 depending on the medium gas and the values for process I are relatively larger than those for process II. Lower limits of the autoionization lifetimes are estimated to be 4.7×10−8 and 1.2×10−10 s for the intermediate ions involved in processes I and II, respectively.