The paper discusses the evaluation of performance characteristics of emissions from hazardous waste incinerators with a mobile laboratory. Incinerators are permitted, under RCRA, to burn hazardous waste. Because of the interest in monitoring incinerator performance on a real-time basis, the EPA designed and built a mobile laboratory for on-site monitoring of hazardous waste incinerator emissions and operating parameters. The result, the Hazardous Air Pollutants Mobile Laboratory (HAPML), is equipped with monitors for on-line analysis of inorganic and organic stack gas emissions. The HAPML has demonstrated its usefulness in a field test of an industrial boiler co-firing hazardous waste. The field test showed that combustion efficiency (CE) may have potential for use as an incinerator performance indicator.