During the summer of 1991, as part of the Lake Michigan Urban Air Toxics Study, measurements of atmospheric reactive gases, fine fraction, and size fractionated acidic aerosol samples were taken at two sites (South Haven, MI and on the research vessel, Laurentian). The fine aerosol samples were collected using an annular denuder system (ADS) which allowed quantification of acidic and basic gases, as well as inorganic ions in the fine particle fraction (< 2.5 micrometers). The size fractionated data was obtained using a six stage micro-orifice impactor equipped with ammonia-scrubbing denuders. The ADS aerosol results showed extreme episodic behavior which correlated well with air mass transport from the southwest. The maximum concentrations observed in South Haven after over lake transport from the southwest were 241 nmol/cu m for aerosol strong acidity (H(+1)), and 3.8 ppb for nitric acid. These elevated acid levels were accompanied by hourly maximum O3 concentrations of 128 and 153 ppb, respectively. Levels at South Haven and aboard the Laurentian were very similar for most of the species measured. Size fractionated particulate mass results also compared well for most species, and showed a typical size dependent behavior. Measurements of aerosol acidity are also compared to those taken in Ann Arbor, MI during the one month study.