N,N-diethylaminobenzaldehyde (DEAB) is a commonly used “selective” inhibitor of aldehyde dehydrogenase isoenzymes in cancer stem cell biology due to its inclusion as a negative control compound in the widely utilized Aldefluor assay. Recent evidence has accumulated that DEAB is not a selective inhibitory agent when assayed in vitro versus ALDH1, ALDH2 and ALDH3 family members. We sought to determine the selectivity of DEAB toward ALDH1A1, ALDH1A2, ALDH1A3, ALDH1B1, ALDH1L1, ALDH2, ALDH3A1, ALDH4A1 and ALDH5A1 isoenzymes and determine the mechanism by which DEAB exerts its inhibitory action. We found that DEAB is an excellent substrate for ALDH3A1, exhibiting a V_max/K_M that exceeds that of its commonly used substrate, benzaldehyde. DEAB is also a substrate for ALDH1A1, albeit an exceptionally slow one (turnover rate ∼0.03 min⁻¹). In contrast, little if any turnover of DEAB was observed when incubated with ALDH1A2, ALDH1A3, ALDH1B1, ALDH2 or ALDH5A1. DEAB was neither a substrate nor an inhibitor for ALDH1L1 or ALDH4A1. Analysis by enzyme kinetics and QTOF mass spectrometry demonstrates that DEAB is an irreversible inhibitor of ALDH1A2 and ALDH2 with apparent bimolecular rate constants of 2900 and 86,000 M⁻¹ s⁻¹, respectively. The mechanism of inactivation is consistent with the formation of quinoid-like resonance state following hydride transfer that is stabilized by local structural features that exist in several of the ALDH isoenzymes.