This paper examined the photolysis of gaseous butyl acetate by built-in microwave discharge electrodeless lamps. Bromine and iodine lamps were used to investigate the effects of microwave input power, inlet concentration, gas residence time, and different filling materials on butyl acetate decomposition and energy yield. Water vapor and ammonia were utilized as additional chemicals to determine if they can improve removal efficiency. Built-in structure and external structure were also compared. When the gas flow velocity is 0.025 m/s, and the inlet concentration is 1.5 mg/m(3), the removal efficiency of butyl acetate can reach over 78%, which is the highest in this study. Additional water vapor and ammonia can hardly improve the degradation. To some extent, built-in MDELs are better than external MDELs. The results will help promote the study of microwave discharge electrodeless lamps and their effect on the degradation of gaseous pollutants. Maybe, as the research goes on, it could realize the span from laboratory scale to practical application, and will become an advanced technology for the gaseous pollutants control.