The effect of p2H on the chemical shifts of two adenine protons and one reduced nicotinamide proton of β-NADH have been examined. The adenine protons had a single pK at a p2H 3.85. The reduced nicotinamide proton showed no pK in the range p2H 3.00 to p2H 7.00. At low p2H values a new peak appeared in the nuclear magnetic resonance spectrum 0.31 p.p.m. downfield from the nicotinamide C-2 proton of β-NADH, and the nicotinamide C-2 proton peak gradually disappeared. It is suggested that this new peak is responsible for some of the anomalies in published results. The alterations in the ultraviolet spectra also indicate a change in the structure within the reduced nicotinamide moiety of β-NADH at low p2H values.