A new method (ROxLIF) for the measurement of atmospheric peroxy radicals (HO2 and RO2) was developed using a two-step chemical conversion scheme and laser-induced fluorescence (LIF) for radical detection. Ambient air is sampled into a differentially pumped flow reactor, in which atmospheric ROx radicals (=RO2+RO+HO2+OH) are chemically converted to HO2 by a large excess of NO and CO at reduced pressures (ROx mode). When only CO is added as a reagent, the sum of atmospheric HO2+OH is converted to HO2 (HOx mode). At the reactor outlet, part of the air flow is transferred into a low-pressure detection chamber, where the HO2 is further converted by reaction with NO to OH, which is then detected with high sensitivity by LIF at 308 nm. The ROxLIF technique has been implemented in an existing LIF instrument that is also capable of measuring atmospheric OH. From the concurrent measurements of ROx, HOx and OH, concentrations of HO2 and RO2 can be determined. The system is calibrated using the quantitative photolysis of water vapor at 185 nm as a radical source. Addition of CO or hydrocarbons to the calibration gas yields well-defined concentrations of HO2 or RO2, respectively, providing an estimated accuracy for the calibration of about 20%. The ROxLIF technique is extremely sensitive and- has detection limits (signal-to-noise ratio=2) of about 0.1 pptv of HO2 or RO2 at a time resolution of 1 min. The paper describes the technique and its calibration, discusses the chemistry in the conversion reactor and possible interferences, and gives an example of ambient air measurements to demonstrate the performance of the new technique.