In aqueous solution, N-substituted isoxazolin-5-one derivatives, which occur in high amounts in seedlings of the tribe Vicieae, can be shown to undergo a proton exchange at C-4, indicative of their aromatic character. Under UV-light these different isoxazolin-5-one derivatives are then broken down, each following the same degradation scheme, some of them forming the well-known toxins of the genus Lathyrus. The loss of aromatic character and the addition of two water molecules is proposed as the underlying mechanism for this photolysis. The biochemical relation between isoxazolin-5-ones and the Lathyrus toxins is discussed.