The hydroxyl radical (OH) controls the lifetimes and therefore the concentrations of many important chemical species in Earth's lower atmosphere including several greenhouse and ozone-depleting species. Two completely different measurement techniques were used in an informal intercomparison to determine tropospheric OH concentrations at Fritz Peak Observatory, Colorado, from 15 July to 24 August 1991. One technique determined OH concentrations by chemical analysis; the other used spectroscopic absorption on a long path. The intercomparison showed that ambient OH concentrations can now be measured with sufficient sensitivity to provide a test for photochemical models, with the derived OH concentrations agreeing well under both polluted and clean atmospheric conditions. Concentrations of OH on all days were significantly lower than model predictions, perhaps indicating the presence of an unknown scavenger. The change in OH concentration from early morning to noon on a clear day was found to be only a factor of 2.