Measurements of atmospheric monoterpene hydrocarbons were made at a site in the Colorado mountains. The research was undertaken to examine the influence of the compounds on the photochemistry of the troposphere. A sampling technique was developed using Tenax GC porous polymer traps with analysis by capillary gas chromatography and detection by flame ionization and mass spectrometry. Positive identification of six monoterpene hydrocarbons, α-pinene, camphene, β-pinene, myrcene, Δ-3-carene, and d-limonene, was obtained, as well as tentative identification of α-thujene and b-plennandrene. A definite seasonal trend was evident in the average monoterpene mixing ratios. The summertime average was 0.30 ppbv for the sum of the five major identified monoterpenes, β-pinene, α-pinene, Δ-3-carene, camphene, and d-limonene, with a high degree of constancy in relative ratios of each throughout the summer months. Wintertime measurements gave mixing ratios below the detection limits (0.001 ppbv of an individual compound). Simultaneous measurements of ozone, NO, NO2 and monoterpene hydrocarbons allowed examination of the contribution of monoterpene photooxidation to ozone production. Based on reported modeling studies, monoterpenes were estimated to be a small source of ozone, insufficient to account for the relatively high ozone mixing ratios (>80 ppbv) sometimes observed at this sampling site.