An extensive study of tropospheric photochemical trace species, including the hydroxyl radial OH was conducted in the Colorado Rocky Mountains from early August to early October 1993. Many of the parameters that influence the abundance of OH in the atmosphere, including j(O3 and NO2), formaldehyde, nonmethane hydrocarbons, NOx, ozone, H2O, aerosols, and others, were measured simultaneously with OH. Instrumentation for measurement of OH included a long-path spectroscopic technique and several in situ techniques: ion-assisted mass spectrometry, laser-induced resonance fluorescence, and a liquid scrubber/liquid chromatographic method. Instrument comparisons between long-path and in situ measurements were conducted for OH, O3, CH2O, NO2, and H2O, and these measurements provide a valuable insight into the chemistry of the region. The complete data set provided a robust set of inputs to a zero-dimensional model for exploring OH photochemistry. Independently measured OH concentrations were in agreement within one standard error much of the time, thus giving confidence that hydroxyl is being measured correctly. OH concentrations were typically low during clean continental airflow with concentrations rarely above 4¿106 cm-3. During occasions of polluted airflow from the Denver-Boulder metropolitan areas, values of OH rose as high as 1.2¿107 cm-3. The nearly complete suite of trace gases measured simultaneously with the hydroxyl radical severely constrained the models and provided numerous opportunities to compare relations between species (e.g. OH versus NOx, OH/HO2, RO2 versus NOx). Modeled hydroxyl results using the complete species suite were about a factor of 1.5 higher than measured OH concentrations suggesting that the photochemistry may not be well understood. The campaign data provide new insights into the chemistry of the lower troposphere. This paper provides an overview of the campaign and brief descriptions of results from individual experiments. Details are provided in the accompanying papers in this issue.¿ 1997 American Geophysical Union