The reaction HO2 + HO2 → H2O2 + O2(1) has been studied at 100 Torr and 222 K to 295 K. Experiments employing photolysis of Cl2/CH3OH/O2/N2 and F2/H2/O2/N2 gas mixtures to produce HO2 confirmed that methanol enhanced the observed reaction rate. At 100 Torr, zero methanol, k1 = (8.8 ¿ 0.9) 10-13 ¿ exp<(210 ¿ 26)/T> cm3 molecule-1 s-1 (2σ uncertainties), which agrees with current recommendations at 295 K but is nearly 2 times slower at 231 K. The general expression for k1, which includes the dependence on bath gas density, is k1 = (1.5 ¿ 0.2) ¿ 10-12 ¿ exp<(19 ¿ 31)/T> + 1.7 ¿ 10-33 ¿ ¿ exp<1000/T>, where the second term is taken from the JPL00-3 recommendation. The revised rate largely accounts for a discrepancy between modeled and measured 2O2> in the lower to middle stratosphere.