The determination of the concentration of OH in the Earth's troposphere is of fundamental importance to an understanding of the chemistry of the lower atmosphere. Although many experiments to measure OH concentration have been performed in recent years, very few operate at sensitivities necessary to measure the extremely low amount of OH in the clean troposphere (0.1--0.2 parts per trillion by volume at summertime local noon). This paper describes an informal intercomparison campaign held at Fritz Peak, Colorado, in summer 1991 to intercompare the OH concentrations determined from a spectroscopic instrument and an in situ chemical conversion instrument, both with sensitivities at or below 5--105 molecules cm-3. Ancillary measurements including those of O3, CO, NO, NO2, NOy, H2O, SO2, aerosols, solar flux, and meteorological parameters were also performed to test photochemical theories of OH formation. These measurements also provided a means for comparing air masses at the long path and in situ sites. The intercomparison was very successful with measured values of OH concentration in agreement within one standard error much of the time. OH concentrations were typically low, rarely above 4¿106 cm-3, with only slow growth during the morning hours, indicating the possible presence of scavenger species. Model results suggest higher than measured OH concentrations or the presence of scavenger species. ¿ American Geophysical Union 1994