The kinetics of the reaction 2H(2)S(aq) + S2O32- + 2H(+) --> 1/2S(8)(s) + 3H(2)O has been investigated at 0-40 degrees C in the pH interval from 5 to 7. The rate law was found to be r = -d[S2O32-]/dt = k[H2S][S2O32-], where k = 8 x 10(11) exp(-74 300/RT) M-1 s(-1). Thermodynamic calculation shows that the reaction is unfavorable at pH greater than or equal to 8, which is confirmed by the experimental observation. The ionic strength effect on the reaction also has been studied. The mechanism postulated to account for the observed rate law involves the formation and subsequent reactions of an intermediate H2S3O32-.