The environmental degradation of a mixture of domoic acid (DA) and kainic acid (KA) in seawater with and without added transition metals is reported. The association constants for kainic acid with Fe (III) and Cu (II) were determined using (1)H nuclear magnetic resonance (NMR; K1,Fe(III) = 2.27 x 10(12), K2,Fe(III) = 8.99 x 10(8), K1,Cu(II) = 1.38 x 10(10), and K2,Cu(II) = 4.35 x 10(7)). The photochemical half-life of kainic acid has been determined to be significantly longer (40-100 h) than that of domoic acid in corresponding marine systems (12-34 h). The significance of this finding was highlighted by a comparison of the quantification of a mixture of kainic and domoic acids during photodegradation by liquid chromatography-tandem mass spectrometry (LC-MS/MS) techniques and the widely used competitive enzyme-linked immunosorbent assay (cELISA; Biosense Laboratories) method. The MS-based analysis showed that approximately 50% of the DA was photodegraded within 15 h. In contrast, the domoic acid cELISA assay reported that the concentration essentially remained unchanged over this period. The possibility of interference from naturally occurring kainic acid during cELISA measurements could lead to the overestimation of total domoic acid, especially if they occur in mixtures in sunlit waters.