Abstract 2, 4-Dihydroxy-7-methoxy-2H-1, 4-benzoxazin-3 (4H)-one (DIMBOA) is synthesized in maize from anthranilic acid-d 4 with high retention of deuterium. Retention of deuterium is only consistent with a mechanism of oxidative decarboxylation of o-carboxyanilinoribulose-5- phosphate or other anthranilate derivative via an intermediate in which oxygenation occurs at the carboxyl-bearing ortho carbon before decarboxylation.