As a laboratory exercise for a course entitled “Isotope Tracers in the Environment,” we brewed beer with fermentable sugar sources that were from both C-3 and C-4 plants and thus had distinct δ13C values (−25.6‰ and −9.8‰ respectively). We measured the δ13C value of CO2 produced throughout the course of beer fermentation and observed a 6.8‰ decrease in the δ13C of the CO2 after about 30 hours. This δ13C change is due to exhaustion of the easily fermentable carbohydrates in the corn sugar starting material (glucose) and the later dominance of fermentation of the less easily metabolized sugars maltose and maltotriose found in malted barley extract. Students were instructed to write a “research paper” with this data set. This laboratory introduced students to techniques and concepts used in stable-isotope biogeochemistry.