Recreational marine engine operation effects water quality as well as air quality. Significant quantities of hydrocarbons are discharged into the rivers, lakes, and estuaries used as recreational boating waters. In order to investigate the impact of recreational marine engine operation on water quality, a MerCruiser 3.0LX four-cylinder four-stroke inboard engine and a Mercury 650 two-cylinder two-stroke outboard engine were tested using EPA required certification procedures. Both engines were tested with exhaust gas/cooling water mixing (scrubbing) in the exhaust stream using both freshwater and saltwater. Additionally, the inboard engine was tested without exhaust scrubbing.Gaseous emissions (HC, NOX, CO, and CO2) from the engines were continuously measured using a constant volume sampling system. Both exhaust gas and cooling water samples were collected and speciated for hydrocarbon species present. In addition, carbonyl compounds were collected by diverting a portion of the exhaust stream through dinitrophenylhydrazine (DNPH) charged cartridges. Chromatography methods were used for species identification.Detailed descriptions of the testing apparatus, equipment, and analysis procedures used are included. Results for gaseous emissions, carbonyl compounds, and aqueous samples are reported. The results show that concerns over HC species entering the aqueous environment are justified.