Emissions from gas- and oil-fueled gas turbines and reciprocating engines for electricity generation and industrial applications are assessed in this report. The assessment method involved a critical examination of existing emissions data, followed by the conduct of a measurement program to fill data gaps based on a phased sampling and analysis strategy. In the first phase of the measurement program, one gas-fueled gas turbine, five distillate-oil fueled gas turbines, and five diesel engines were selected for testing. Evaluation of test results led to the recommendation for additional tests to determine SO3 and organic emissions from diesel engines which were subsequently conducted at three of the diesel engine sites previously tested. The results of the emissions assessment indicate that internal combustion sources contribute significantly to the national emissions burden. The source severity factor, defined as the ratio of the calculated maximum ground level concentration of the pollutant species to the level at which a potential environmental hazard exists, was used to identify pollutants of environmental concern.