Ambient concentrations of the hydroxyl (OH), hydroperoxyl (HO2), and total peroxy (&Sgr;RO2) radicals were measured as part of the Tropospheric OH Photochemistry Experiment at Idaho Hill, Colorado, during August and September of 1993. OH radicals were measured using ion-assisted mass spectroscopy and low-pressure laser-induced fluorescence (LIF) detection techniques. HO2 was measured using chemical conversion and LIF detection of OH. &Sgr;RO2 radicals were measured using a chemical amplifier technique. The simultaneous measurements of these key species provide an opportunity to test our present understanding of the fast photochemistry of the troposphere. Measured HO2/OH ratios were typically between 15 and 80, and agreed well with predictions under conditions where NO mixing ratios were greater than 100 pptv. However, under clean conditions the measured ratio was a factor of 3--4 lower than predicted. The RO2/HO2 ratio was typically a factor of 4--15 larger than predicted by present theories of tropospheric chemistry. A steady state model was used in an attempt to analyze the discrepancies between the measured HO2/OH and RO2/HO2 ratios with present theories of hydrocarbon oxidation in the troposphere.¿ 1997 American Geophysical Union