An upper-level mechanistic organic experiment is outlined where undergraduates measure kinetic rate constants for decarboxylation of pyrrole-2-carboxylic acid by the initial-rates method. UV spectroscopy is used to monitor reactant disappearance in both hydrochloric acid and deuterium chloride at different temperatures. Individual data are pooled and utilized by the class to calculate solvent kinetic isotope effects and reaction activation parameters (the latter from Eyring plots). Students interpret experimental results and additional literature data to deduce likely operative decarboxylation mechanisms at varying acidities. (Contains 2 figures and 1 scheme.)