Domoic acid ([2S-[2 alpha,3 beta,4 beta(1Z,3E,5S*)]]-2-carboxy-4-(5-carboxy-1-methyl-1,3-hexadienyl)-3-pyrr olidineacetic acid, DA), a harmful algal toxin produced by several microorganisms genus involved in red tides, was tested for degradation in de-ionized and synthetic sea water using Fenton and Fenton-like advanced oxidation processes (AOPs). Four different AOPs-Fenton, photo-Fenton, cobalt/peroxymonosulfate and cobalt/peroxymonosulfate/UV-were tested. Cobalt/PMS/UV demonstrated the highest initial reaction rate when compared with the other oxidation processes tested. Results obtained from tests performed in synthetic sea water were similar for the case of Co/PMS/UV process whereas important decrease in degradation reaction rate was observed for photo-Fenton reaction probably due to the formation of chlorine-Fe2+ or sulfate-Fe2+ complexes.