The H02+ H02 and D02+ D02 reactions in the gas phase have beenstudied by the flash photolysis/UV absorption technique. Rate constants were measured at pressures between 100 and 700 torr of Ar and N2 at temperatures between 230 and 420 K, and with up to 10 torr of added water vapor The overall disproportionation rate constant can be expressed as the sum of pressure-independent and pressure-dependent terms. For the H02+ H02 reaction, k¡= 2.3 X 10" 13 exp (600/7)+ 8.4 X 10" 34 [M] exp (l 100/7) cm1 23 molecule'1 s'1 for M= Ar and kx= 2.2 X 10" 13 exp (620/7)+ 1.9 X 10'33 [M] exp (980/7) cm3 molecule'1 s'1 for M= N2. For the D02+ D02 reaction, k2= 2.2 X 10'14 exp (900/7)+ 3.6 X 10'34 [M] exp (1200/7) cm3 molecule" 1 s'1 for M= N2. The experimentaluncertainty in kx and k2, including systematic errors, is 25%(one standard deviation). The enhancement of the H02+ H02 rate constant in the presence of added water vapor was studied over the temperature range 250-298 K and found to contribute a multiplicative term (1+ 1.4 X 10'21 exp (2200/7)[H20]) to the rate constant expressions. The mechanism of the H02+ H02 reaction was investigated with a two-channel RRKM model which suggested that the binding energy of the H04H intermediate lies in the range of 12-20kcal mol" 1 relative to H02 reactants.