The report documents the discussion and results of the U.S. EPA Acid Aerosol Measurement Workshop, conducted February 1-3, 1989, in Research Triangle Park, North Carolina. It was held in response to recommendations by the Clean Air Scientific Advisory Committee (CASAC) regarding issues associated with the characterization of aerosol acidity and acid aerosol measurement methods. The workshop was structured to identify appropriate indicators and methodology for characterizing aerosol acidity and to develop ideas and recommendations for the evaluation of acid aerosol methods currently in use. The workshop participants identified the development of an accurate, reliable, and interference-free method as an important initial research objective, and concluded that the most appropriate indicator of aerosol acidity is fine-particle strong acidity measured as hydrogen ion by either pH or titration. After considering procedures to evaluate current acid aerosol measurement methods, workshop participants concluded that the evaluation process must include the development and distribution of audit standards to check the accuracy and precision, and both laboratory and field tests to evaluate and compare acid aerosol sampling and analysis systems. The report provides background on the CASAC recommendations and summarizes the presentations, discussions, conclusions, and recommendations at the workshop.