The second in a series of Atmospheric Environment Service field studies in the Athabasca Oil Sands area was carried out during 3-13 February 1977. The objectives of the study were the same as those of the March 1976 study, namely to obtain information on the meteorology of the area and/or the dispersal and behaviour of the Great Canadian Oil Sands (GCOS) plume. In addition, the study was extended to determine the rate of SO2 oxidation of the GCOS plume and to determine its constituents. As a consequence, the AES experimental program consisted of wind, temperature, and humidity measurements as a function of height using balloon-borne minisondes and radiosondes tracked by theodolities and a tethersonde-borne instrument; photography of the GCOS plume; and aircraft sampling of SO2, sulphate, and sulphuric acid in the GCOS plume downwind to about 30 km from the stack. For the majority of the experiments, surface inversion conditions were observed. The meteorological structure at Lower Syncrude Site and at Syncrude Mine Site is discussed. Typically, with a cross-valley flow, the wind at the lower site was decoupled from the winds aloft, with the lower level flow aligning with the valley walls. The wind at the upper site showed a similar shifting of the surface winds to align along the valley direction. These results suggest that during stable conditions, the flow through the surface inversion is strongly influenced by the broad basin of the Athabasca River.