The rate constants of the reactions H2O2 + M -> 2OH + M (1) OH + H2O2 -> H2O + HO2 (2) were measured in shock-heated H2O2/Ar mixtures using laser absorption diagnostics for H2O and OH. Time-histories of H2O were monitored using tunable diode laser absorption at 2550.96 nm, and time-histories of OH were achieved using ring dye laser absorption at 306 nm. Initial H2O2 concentrations were also determined utilizing the H2O diagnostic. On the basis of simultaneous time-history measurements of OH and H2O, k(2) was found to be 4.6 x 10(13) exp(-2630 K/T) [cm(3) mol(-1) s(-1)] over the temperature range 1020-1460 K at 1.8 atm: additional measurements of k(2) near 1 atm showed no significant pressure dependence. Similarly, k(1) was found to be 9.5 x 10(15) exp(-21 250 K/T) [cm(3) mol(-1) s(-1)] over the same temperature and pressure range.