An instrument to detect atmospheric concentrations of the hydroxyl (OH) and hydroperoxyl (HO ₂ ) radicals has been developed using the FAGE (fluorescence assay by gas expansion) technique. The instrument is housed in a mobile laboratory and monitors the OH radical via on-resonance laser-induced fluorescence (LIF) spectroscopy of the A ² Σ ⁺ (v′=0)–X ² Π _i (v ^″ =0) transition at ca. 308 nm. Ambient air is expanded through a 1 mm nozzle to low pressure where it is irradiated by the laser pulse at a repetition rate of 7 kHz, with the resultant fluorescence being detected by gated photon counting. HO ₂ is monitored by chemical conversion to OH by the addition of NO, with subsequent detection using LIF. Following laboratory and field calibrations to characterise the instrument sensitivity, detection limits of 1.8×10 ⁶ and 2.1×10 ⁷ molecule cm ^-3 were determined for OH and HO ₂ respectively, for a signal-to-noise ratio, S/N, of 1 with 150 s signal integration time. The instrument was deployed for the first time during the ACSOE field campaign at Mace Head, Eire, for which illustrative results are given.