The rapid reaction F + Br → BrF + Br has been used, with detection of BrF by laser-induced fluorescence, as a monitor of F-atom concentrations in a discharge-flow experiment. Absolute rate constants for the hydrogen-abstraction reactions F + H2, CH2Cl2, CH2ClF and CH2F2 were measured to be k298=(2.6 ± 0.1)× 10–11, (8.9 ± 0.6)× 10–12, (7.8 ± 0.4)× 10–12 and (5.7 ± 0.3)× 10–12 cm3 molecule–1 s–1, respectively. (The errors given are 1σ; a further 10–15% should be added to account for systematic errors.) The reaction of F with CHCl2 was modelled to give k298=(3.4+1.7–2.5)× 10–10 cm3 molecule–1 s–1.