Using an indirect technique to obtain a time-integrated signal, we have detected and quantified the hydroxyl radical (OH) in a commercial building. For the purpose of the measurements, the conditions in the office setting were manipulated, but they were representative of conditions that occur naturally. During monitoring periods when the concentrations of both ozone (O3) and d-limonene were elevated and the air exchange rate moderate, we found the average indoor OH concentration to be approximately 7 × 105 molecule/cm3. This value is lower than typical outdoor midday values (5 × 106 molecule/cm3) but larger than outdoor nighttime values. The results confirm that reactions among O3 and olefins can generate meaningful quantities of OH under conditions that commonly occur indoors. In turn, OH initiates a complex series of reactions that generate still other radicals and ultimately produce species that can adversely affect human health and artifacts in indoor environments.