Field studies of photosynthesis in Pinus contorta/Pinus banksiana (lodgepole pine/jack pine) hybrids, Picea glauca (white spruce) and Populus tremuloides (aspen) subjected to SO2 and H2S from a nearby natural gas processing plant were initiated near Whitecourt, Alberta, Canada during the summer of 1974. The site was characterized as a Pinus-Picea glauca/Arctostaphylos uva-ursi association (pine-white spruce/bearberry). A 15 m high scaffold was used as access to mid-crown foliage in the pines while the spruce and aspen were accessible from the round. Net assimilation rates, transpiration rates and leaf resistances were calculated and water deficits were monitored. Photosynthetic rates measured were in a low range for the conifers studied, with pine having a maximum of 3.28 mg dm−2 h−1 and white spruce a maximum value of 2.3 mg dm−2 h−1 The low maximum photosynthetic rate determined for aspen is thought to be attributable to the onset of autumn. Chemical analyses for SO4-sulfur using the methylene blue colorimetric method of Johnson and Nishita (1952) showed levels of 300 to 700 ppm, with the older foliage showing slightly higher values. Visible chronic S02 symptoms had a pronounced sun, or upward, orientation. Ambient SOD H2S and total S were measured using a Tracor 270HA Atmospheric Sulfur Analyzer (chromatographic method) and trends in ambient SO2 concentrations using an Envirometrics SO2 Analyzer (polarographic method). Concentration was found to be variable for SO2 and generally below 0.05 ppm. A concentration gradient of SO2 was found to exist in the lodgepole pine/jack pine stand with the SO2 values above the canopy generally higher than below the canopy (0.1 ppm above and 0.05 ppm below). This condition was occasionally reversed. The plant canopy is considered to act as a barrier to downward diffusion of the S emissions in the first case and also a barrier to upward diffusion of S emissions present due to advection in the stand in the second case. The vegetative environment surrounding the Windfall Gas Plant is definitely affected by S gas emissions but the extent remains to be determined.