This laboratory exercise involves the quantitative analysis of dissolved, particulate and total organic carbon concentrations in environmental samples. Students learn the principle of the analytical method (oxidation of organic carbon to carbon dioxide) and instrumentation (UV-persulfate, low temperature oxidation and infrared absorbance). Although this is a very basic experiment, TOC measurements can be very difficult to make accurately. They learn to evaluate their results critically relative to common interferences for this method and common types of experimental errors. Finally, this exercise serves as an important basis for all the other laboratories in the course, because once the amount of organic carbon in a sample is known, students will seek to characterize various qualitative and functional attibutes among the different organic fractions that make up this complex mixture.