This report describes the results of field and laboratory studies of combined NH/sub 3/ and SO/sub 3/ conditioning at the Big Brown Station of Texas Utilities Generating Company. This unusual combination of conditioning agents is used routinely at the Big Brown Station in order to improve the performance of the cold-side electrostatic precipitators. The primary objectives of this field study were to evaluate the performance of one of the Big Brown precipitators, and to obtain data on the concentration, composition, and size distribution of the fly ash, as well as the composition of the flue gas and the overall and fractional collection efficiencies of the precipitator. The laboratory studies of the Big Brown fly ash were intended to further characterize the ash both physically and chemically, and to study the attenuation of the electrical resistivity of the ash associated with the surface film produced by the dual conditioning process and by the use of SO/sub 3/ conditioning alone. 6 references, 22 figures, 9 tables.