Two different instruments were deployed to measure hydroperoxy radical (HO2) and total peroxy radicals during May and June 2002 at a rural site located at Rock Springs near State College, Pennsylvania. One instrument was the Penn State Ground-Based Tropospheric Hydrogen Oxides Sensor (GTHOS), which measured HO2 by laser-induced fluorescence (LIF) of OH following the chemical conversion of HO2 to OH by adding NO. The other was the Peroxy Radical Chemical Ionization Mass Spectrometer (PerCIMS) instrument that provided HO2 (HO2 mode) or total peroxy radicals (HOxROx mode) measurements. The two instruments were compared by exchanging calibration sources and by side-by-side ambient measurements. Excellent agreement was obtained in the calibration comparisons with the slope of a correlation plot close to 1.0 and a small intercept. Daily HO2 measurements from GTHOS and PerCIMS (in HO2 mode) on 4 days show that diurnal variations normally agreed to within about 40%. A comparison of HO2 measured by GTHOS and HOxROx measured by PerCIMS shows good correlation for both average diurnal variation and individual diurnal cycles. On average, HO2 from GTHOS was about 0.66 times the HOxROx from PerCIMS, meaning that organic peroxy radicals account for about one third of the total peroxy radicals in this environment.