An extensive air quality monitoring program was carried out in the Washington, DC - Baltimore region during the July 14 - August 15 period in 1980. The field study included data collection at numerous ground stations as well as the use of two aircraft sampling platforms. Measurements included ozone, oxides of nitrogen, individual hydrocarbons (C2-C10), condensation nuclei and visual distance plus numerous meteorological parameters. Specific areas of interest in the study included the acquisition of air quality data for (1) developing and testing a regional air quality simulation model and (2) gaining a better understanding of oxidant production and transport in plumes emanating from Washington, DC and Baltimore. The purpose of the report is to describe oxidant behavior downwind of these two cities in cases where plume interaction was observed. The authors have selected seven days during the 1980 study period when southwesterly winds carried the Washington, DC plume into the region occupied by the Baltimore plume. A case study analysis was performed for each of these days and general conclusions have been presented regarding oxidant patterns in the regions where plume interactions occurred.