This paper describes a method for determination of trihalomethanes (THMs) in drinking water via solid-phase microextraction (SPME) GC/MS as a means to develop and improve student understanding of the use of GC/MS for qualitative and quantitative analysis. In the classroom, students are introduced to SPME, GC/MS instrumentation, and the use of MS data for compound identification and quantification through traditional lectures and in-class problem solving. In the lab, they are introduced to GC/MS tuning and method development, learn how to use the GC/MS data system, and apply their knowledge to identify and quantify THMs in tap water samples. The method for this application is relatively simple, fast, and effective, and is based on the use of autosampler, SPME headspace sampling, a GC quadrupole ion trap MS instrument, and fluorobenzene as an internal standard. Data from analyses of standards demonstrated 1 ppb LODs, good linearity in the 5-100 ppb range, RSDs of 6% or lower, and good recoveries from spikes of a tap water sample. This method as described is appropriate for environmental analysis, instrumental analysis, or quantitative analysis courses. It provides students with hands-on experience in the use of SPME-based sampling methods and GC/MS for reliable identification and accurate quantitative analysis at low ppb levels.