One of the major objectives of the 1993 Tropospheric OH Photochemistry Experiment (August 8 to October 4, 1993) was to compare atmospheric measurements of the important trace gas species NO2, O3, and H2O by in situ techniques and by long-path differential absorption spectroscopy. These compounds were measured over a 20.6 km folded optical path from Fritz Peak Observatory to Caribou Mine where a retroreflector array is located. In situ measurements were made at Idaho Hill, 0.7 km northwest of Caribou Mine and 25 km west of Boulder, Colorado. NO2 was determined at Idaho Hill by photolysis followed by NO chemiluminescence. O3 and H2O were measured at this site using calibrated commercial instruments. Using fractional differences (/Idaho Hill) as a measure of agreement, overall agreement for ozone and water are +3¿7% and -4¿17%, respectively. For NO2 the comparison is complicated by the influence of strong sources and large ambient variability during the measurements. During times of clean westerly flow, fractional differences of +30¿110% were observed. Further analysis of the data to evaluate these biases show that for mixing ratios greater than 300 pptv (well above the 1&sgr; detection limit of 30--45 pptv) the two methods differ by no more than 10%, a value well within the combined uncertainties of the measurement techniques.¿ 1997 American Geophysical Union