QOOH radicals are key species in autoignition, produced by internal isomerizations of RO2 radicals, and are central to chain branching reactions in low-temperature combustion. The kinetics of QOOH radical decomposition and reaction with O-2 have been determined as a function of temperature and pressure, using observations of OH radical production and decay following H-atom abstraction from t-butyl hydroperoxide ((CH3)(3)COOH) by Cl atoms to produce QOOH (CH2(CH3)(2)COOH) radicals. The kinetics of QOOH decomposition have been investigated as a function of temperature (251-298 K) and pressure (10-350 Torr) in helium and nitrogen bath gases, and those of the reaction between QOOH and O-2 have been investigated as a function of temperature (251-304 K) and pressure (10-100 Torr) in He and N-2. Decomposition of the QOOH radical was observed to display temperature and pressure dependence, with a barrier height for the decomposition of (44.7 +/- 4.0) kJ mol(-1) determined by master equation fitting to the experimental data. The rate coefficient for the reaction between QOOH and O-2 was determined to be (5.6 +/- 1.7) x 10(-13) cm(3) s(-1) at 298 K, with no significant dependence on pressure, and can be described by the Arrhenius parameters A = (7.3 +/- 6.8) x 10(-14 )cm(3) s(-1) and E-a = -(5.4 +/- 2.1) kJ mol(-1) in the temperature range of 251-304 K. This work represents the first measurements of any QOOH radical kinetics as a function of temperature and pressure.