The reaction rates of ground-state OH radicals, X²Π(v″= 0), with the molecules CH₄, CO, HCl, DCl, HBr and DBr have been investigated in the gas phase by time-resolved resonance fluorescence at λ= 307 nm [OH(A²∑⁺–X²Π, (0, 0)]. The OH radicals were generated photochemically by the repetitive vacuum ultraviolet photolysis of water vapour in a flow system, kinetically equivalent to a static system, using a high-intensity, magnetically pinched pulsed light source, and optically excited to the A²∑⁺ state by means of a resonance source operating in the standard orthogonal arrangement. The resulting time-resolved resonance fluorescence signals were monitored by means of a highly sensitive detection system employing pre-trigger photomultiplier gating, photon counting and signal averaging. Absolute second-order rate constants (k/cm³ molecule^–1 s^–1, 300 K) were obtained for the reactions of OH with the above molecules as follows: CH₄(7.66 ± 0.64)× 10^–15 CO (1.46 ± 0.12)× 10^–13, HCl(6.66 ± 0.52)× 10^–13, DCl (3.48 ± 0.30)× 10^–13, HBr (6.01 ± 0.32)× 10^–12 and DBr (2.05 ± 0.14)× 10^–12, the quoted errors being 2σ. The rate constants for CH₄ and CO constituted a kinetic test of the system, being in accord with accepted values for these quantities. The data for the hydrogen halides are compared, where possible, with previously determined rates obtained from monitoring OH directly by various methods. To the best of our knowledge, the absolute rate constant for the reaction with DBr has not been reported hitherto from direct monitoring of OH.