Dilute guarana´ ("Paullinia cupana") powder aqueous extract is a soft drink that contains three structurally related methylxanthines (MXs), caffeine (CF), theobromine (TB), and theophylline (TP); CF is present in a large excess. Literature data show that values of [lambda][subscript max] and [epsilon][subscript max] of these three MXs are very close. Consequently, quantitative analysis using a single technique (UV-vis) is difficult, at best. We asked our students how to determine the three MXs simultaneously. They suggested use of chromatographic separation followed by mass spectrometry (MS) detection. Use of electrospray ionization solved the MX structure identification problem only partially because T Band TP have the same molar mass. Using LC-MS/MS solved this problem, as TB and TP have distinct retention times and fragmentation patterns. This is an important everyday situation (analysis of foods and drinks)because of the popularity of CF-containing drinks (guarana, coffee, tea, cacao) and health concerns about abuse of CF consumption.