Equilibrium denaturation of the 72 amino acid alpha/beta-protein MerP, by acid, guanidine hydrochloride, or temperature, is fully reversible and follows a two-state model in which only the native and unfolded states are populated. A cis-trans equilibrium around a proline peptide bond causes a heterogeneity of the unfolded state and gives rise to a slow- and a fast folding population. With a rate constant of 1.2 s(-1) for the major fast folding population, which has none of the common intrinsically slow steps, MerP is the slowest folding protein of this small size yet reported.