The stereochemistry-determining step of the self-assembled chiral Bronsted acid-catalyzed aziridination reactions of MEDAM imines and three representative diazo nucleophiles has been studied using ONIOM(B3LYP/6-31G*:AM1) calculations. The origin of cis selectivity in the reactions of ethyldiaz-oacetate and trans selectivity in reactions of N-phenyldiazoacetamide can be understood on the basis of the difference in specific noncovalent interactions in the stereochemistry-determining transition state. A H-bonding interaction between the amidic hydrogen and an oxygen atom of the chiral counterion has been identified as the key interaction responsible for this reversal in diastereoselectivity. This hypothesis was validated when a 3 degrees diazoamide lacking this interaction showed pronounced cis selectivity both experimentally and calculationally. Similar trends in diastereoselection were observed in analogous reactions catalyzed by triflic acid. The broad implications of these findings and their relevance to chiral Bronsted acid catalysis are discussed.