An instrumented helicopter was used to obtain a record of temperatures and sulfur dioxide concentration in the plume from the Keystone Power Plant in western Pennsylvania. The records were then evaluated to determine plume heights under a variety of atmospheric conditions. To assist in determining the plume boundaries, a device was used which detected charged particles in the plume. The equipment and techniques used to obtain the data and the methods employed to reduce it are described. The data from ten flight days are presented. Included are general descriptions of each day, temperature soundings and plume heights and SO2 maximum concentrations obtained from traverses through the plume. Twenty cases from seven flight days were analyzed and evaluated. For the cases analyzed, the average wind speed varied from 4.8 to 12.5 meters per second and the stability from about 0.15 to 1.5 degrees C. per 100 meters.