The kinetics of the disproportionation reactions of OH and OD radicals, OH + OH --> O + H2O (1), OD + OD --> O + D2O (3), have been studied by the mass spectrometric discharge-now method at temperatures between 233 and 360 K and at total pressure of 1 Torr of helium. The following Arrhenius expressions were obtained: k(1) = (7.1 +/- 1.0) x 10(-13) exp[(210 +/- 40)/T] and k(3) = (2.5 +/- 0.5) x 10(-13) exp[(170 +/- 60)/T] cm(3) molecule(-1) s(-1). Reaction 4 between OH and OD radicals was also investigated and the same rate constant as for reaction I was measured. Both the observed temperature dependence of k(1) and the measured kinetic isotopic effect (k(1)/k(3)) are compatible with the mechanism proposed in a previous theoretical study (Harding, L. B.; Wagner, A. F. 22nd International Symposium on Combustion, 1988). In addition, a temperature independent rate coefficient of (1.20 +/- 0.25) x 10(-10) cm(3) moIecule(-1) s(-1) was measured for the reaction D + NO2 --> OD + NO in the temperature range 230-365 K.