Analysis of halogenated volatile organic compounds (HVOCs) by GC-MS demonstrates the use of instrumentation in the environmental analysis of pollutant molecules and enhances student understanding of stable isotopes in nature. In this experiment, students separated and identified several HVOCs that have been implicated as industrial groundwater contaminants and deduced the isotopic content by analyzing the molecular ion region of the mass spectra. The HVOCs were analyzed by direct sampling of the headspace vapor over a dilute aqueous solution, which is designed to simulate a contaminated groundwater sample. The rapid headspace sampling and analysis allowed completion of the experiment by many lab classes per week. The impact of the lab was assessed using online surveys before and after the lab that queried student perceptions of their understanding, skills, and attitudes toward science and chemistry. The surveys showed noticeable gains in understanding and skills; however, few changes in attitudes were seen among students, the majority of whom already seemed committed to science and engineering careers and majors. (Contains 2 figures.)