Monitoring resonance absorption at the Q(1)(4) line near 308.4 nm, HO concentration-time profiles were recorded during the thermal decomposition of H2O2 in shock waves between 930 and 1680 K. The results were analyzed with respect to the reactions HO+H2O2-->H2O+HO2 (2) and HO+HO2-->H2O+O-2 (3). Above 800 K, reaction (2) shows a strong increase of the rate constant with temperature. Over the range 240 less than or equal to T less than or equal to 1700 K the apparent rate constant is represented as k(2) = [1.7 . 10(18). exp(-14 800 K/T)+2.0 . 10(12) exp (-215 K/T)] cm(3) mol(-1) s(-1). The rate constant k(3) decreases from a value of 2.0 . 10(13) at 1100 K to a minimum value of 1.1 . 10(13) at 1250 K, and rises again to a value of 4.5 . 10(13) cm(3) mol(-1) s(-1) at 1600 K. The anomalous temperature dependences of k(2) and k(3) suggest mechanisms involving intermediate complex formation.