Hydroxyl (OH), hydroperoxy (HO₂) radicals, collectively known as HO _x , and OH reactivity, were measured during the PMTACS–NY (PM2.5 Technology Assessment and Characteristics Study-New York) summer 2002 intensive at Whiteface Mountain, Wilmington, New York. The measurement results of OH and HO₂ for 4 weeks are presented. Diurnal cycles show that the average noontime maximum mixing ratios were about 0.11 pptv (2.6 × 10⁶ cm⁻³) for OH and 20 pptv for HO₂. Measured HO₂ to OH ratios were typically between 40 and 400, which are greater than those obtained in polluted and semipolluted rural environments. Low but significant mixing ratios of OH and HO₂ persisted into early evening and were frequently observed during nighttime, consistent with previous studies in different environments. Steady state OH and HO₂ were calculated with a zero-dimensional chemical model using a complete Regional Atmospheric Chemical Mechanism (RACM) and a parameterized RACM which was constrained to the measured OH reactivity. Good agreement was obtained between the complete RACM and the parameterized RACM models. On average, the complete RACM model reproduced the observed OH with a median measured-to-modeled OH ratio of 0.82 and daytime HO₂ with a median measured-to-modeled HO₂ ratio of 1.21. The reasonably good agreement in this study is inconsistent with the significant underestimation of OH in the Program for Research on Oxidants: Photochemistry, Emissions, and Transport in 1998 (PROPHET98) study at a similar forested site. HO _x budget analysis indicates that OH was primarily from the photolysis of HONO and O₃ during the day and from O₃ + alkenes reactions at night. The main HO _x loss was the self reaction of HO₂. The good agreement between the measured and calculated OH reactivity in this environment contrasts with findings in the PROPHET2000 study, in which significant OH reactivity was missing and the missing OH reactivity was temperature-dependent.