The kinetics of the peroxynitrite (ONOO- and HOONO, PN for both) reaction with (Me)(2)X (X = S and Se, dimethyl sulfide (DMS) and dimethylselenide (DMSe)) was studied. The reaction is first order with respect to both PN and (Me)(2)X concentrations. (Me)(2)X=O and NO2- were formed with 100% yield based on PN in all cases except in the DMS reaction with HOONO, where the yield was similar to80%. DMSe is much more reactive than DMS toward both ONOO- and HOONO, while HOONO reacts much faster than ONOO- with both (Me)(2)X. The activation entropies (in cal deg(-1) mol(-1)) and enthalpies (given in parentheses, in kcal/mol) for the reactions of DMS and DMSe with HOONO were determined to be -23 +/- 3 (5.9 +/- 0.5) and -20.5 +/- 3 (4.6 +/- 0.5), respectively. The corresponding values for the reaction with ONOO- were -29 +/- 3 (9.7 +/- 0.6) and -28 +/- 3 (7.2 +/- 0.5). A partial replacement of water with an organic cosolvent resulted in a decrease in the rate of reaction of ONOO- with both DMS and DMSe, while it had no effect on the rates of reaction with HOONO. The experimental data were analyzed using the reaction mechanism based on our previous computational studies (Musaev, D. G.; Geletii, Yu.V.; Hill, C. L. J. Phys. Chem. A 2003, 170, 5862). This mechanism includes the formation of a prereaction complex followed by O-O bond cleavage in a rate-limiting step. Strong hydration of ONOO- decreases the O-O bond cleavage barrier in a transition state, resulting in a lower reaction rate in mixed aqueous-organic solvents. In general, a fairly quantitative agreement between experimental and theoretifcal data was obtained.