Some N-substituted proline derivatives catalyse the reactions of alkyl aldehydes with electrophilic partners, and others give negligible amounts of product. The key reaction between the aldehyde and the proline derivative has been studied, and a rationale for the reactivity observed is presented. Simple proline amides derived from aromatic amines (poor catalysts) form stable imidazolidinones quite rapidly at the expense of enamines, the latter being detected, if at all, for less than an hour. Less basic proline amides form a kinetic diastereomer of an imidazolidinone that very slowly inverts its stereochemistry to give a final thermodynamic product. The stereochemistry of these diastereomers was fully assigned by 1H NMR, NOE, and TOCSY experiments. A proline amide derived from an alkylamine (good catalyst) forms high concentrations of enamine that is only slowly (days) converted into a single diastereomer of an inert imidazolidinone. In contrast, another good catalyst, a proline sulfonamide, immediately forms a reactive imidazolidinone, that rapidly ring-opens and exchanges with other aldehydes. This presumably drip-feeds iminium ions and enamines into the catalytic cycle. Thus, there are two quite different mechanistic regimes that lead to efficient catalysis. These mechanistic insights should now allow some element of rational design of prolinamides for enamine reactions using aldehydes.