SUMMARY— The characteristic seasoning flavor of hydrolyzed vegetable proteins has previously been attributed to α‐ketobutyric acid formed from threonine under the conditions of acid‐hydrolysis. However, pure α‐ketobutyric acid is virtually odorless and tasteless, and it develops the characteristic flavor only upon ageing. The true flavoring principle is probably α‐hydroxy‐β‐methyl‐Δ.α,β‐γ‐hexenolactone, which is formed from the acid by dimerization, lactonization, and decarboxylation. This conversion occurs spontaneously, but is greatly accelerated under acidic conditions.