The rates of the reactions of OH with H2, D2, HCl and DCl have been determined between 210 and 460 K using time-resolved resonance absorption to monitor removal of OH following creation of the radicals by flash photolysis of mixtures containing H2O or N2O + H2. Arrhenius rate laws are obeyed by all four of these reactions throughout this range of temperature: kOH + H2= 1.8 × 10–11 exp[–19.4 kJ mol–1/RT] cm3 molecule–1 s–1, kOH + D2= 1.25× 10–11 exp[–21.5 kJ mol–1/RT] cm3 molecule–1 s–1, kOH + HCl= 4.1 × 10–12 exp[–4.4 kJ mol–1/RT] cm3 molecule–1 s–1, kOH + DCl= 4.7 × 10–12 exp[–6.5 kJ mol–1/RT] cm3 molecule–1 s–1. The observed kinetic isotope effects are compared with predictions based on 3- and 4-atom models of the transition states using semi-empirical potential functions to estimate the transition state properties. Preliminary measurements on the reactions of OH with HBr and HI are also reported. The rate constants at 298 K are respectively 4.5 × 10–12 and 9 × 10–12 cm3 molecule–1 s–1.