High-frequency 1H NMR was used to study the structure of the adducts which form when various carbonyl compounds react with pyridine nucleotides at elevated pH. The adducts of N-(2,6-dichlorobenzyl)nicotinamide-acetone, N-(2,6-dichlorobenzyl)nicotinamide-pyruvate, NMN-pyruvate, NAD-pyruvate, NAD-acetaldehyde and NAD-oxaloacetate form with identical structural features and configuration. The adducts are pyridine N-4-substituted compounds. A second 6-membered ring forms by addition of the nicotinamide amido to the carbonyl group of the compound forming the addition complex. Cyclization occurs stereospecifically, indicating that the stereochemistry is predetermined by the initial attack at the N-4 position. Two diastereomeric forms are observed for each nucleotide adduct. The determination of configuration at all symmetric C atoms of these adducts is discussed.