A decrease in absorbance at 340 nm, at rates similar to those obtained with alcohol dehydrogenases in routine assays, occurred when NADH or NADPH was mixed with acetaldehyde that had been exposed to air for various durations. NAD(P)H was apparently oxidized by interfering substance(s) present in acetaldehyde. Reagent-grade acetaldehyde from newly opened bottles as well as acetaldehyde redistilled under strictly Ol-free conditions contained minimal amounts of NAD(P)H-reacting substance(s). Redistillation under poor anaerobic conditions or in air increased the amount of NAD(P)H-reacting substance(s) in redistilled acetaldehyde. NADPH reacted at a higher rate than NADH with the interfering substance(s) in Tris-Cl buffer at pH 7.5. Also, the reaction was faster in Tris buffer than in phosphate buffer at pH 7.5. The NAD(P)Hoxidizing reaction may not be apparent when the nominal concentration of acetaldehyde used was below 5 mM, but the measured ethanol dehydrogenase activity could be significantly lower with acetaldehyde containing a measurable level of interfering substance(s). This study suggests that acetaldehyde is most easily tested with NADPH for the presence of a significant level of interfering substance(s) and that redistillation, if necessary, must be performed under strictly &free conditions.