The specific accumulation of vanadium by certain ascidians, known as sea squirts or tunicates, has been of long-standing interest. Since vanadium is in the +5 oxidation state in sea water but in the +3 state in ascidian blood cells, some reducing agent must participate in the accumulation process. Tunichromes, a class of hydroxy-dopa containing tripeptides, have been significantly studied as a candidate of the reducing agent in ascidians. Although Ryan et al. explained their EPR results as indicating that tunichromes would reduce vanadium(V) and (IV) to the +3 oxidation sate in water, direct evidence for the reduction to the +3 state by tunichromes has not yet been obtained. Furthermore, the finding that vanadium and tunichrome are located in separate blood cells has raised further questions as to the participation of tunichromes in the vanadium reduction process. Frank et al. have shown that ascidian blood cells contain an aliphatic sulfonic acid and a disulfide together with vanadium(III) ions. Since these sulfur compounds are formed by the oxidation of thiols, Frank's observation indicates that a thiol would be another candidate as a reducing agent in ascidians. However, in simple inorganic systems, thiols are only able to reduce V(V) to V(IV) and not further to V(III). Therefore, some chelating agent should be required to stabilize the +3 oxidation state and as a result allow easier access to the +3 oxidation state. We investigated if thiols can reduce vanadium(IV) to (III) in water in the presence of amino polycarboxylates.Na 2 H 2 edta 2H 2 O (0.26 g, 0.7 mmol) and 0.22 g (1.0 mmol) of VOSO 4 3H 2 O were dissolved in 15 cm 3 of water. To the solution was added an aqueous solution (15 cm 3 ) of L-cysteine methyl ester (L-cysme) hydrochloride (0.34 g, 2.0 mmol). The solution was immediately deaerated and filled with argon. The pH of the solution was adjusted to 6.5. The color slowly changed to a dark reddish brown. In the course of this reaction, the visible absorption bands at 580 and 770 nm became weak and new bands appeared around 450 and 530 nm (FIG. 1). After about 4 hours, another portion of Na 2 H 2 edta (0.11 g, 0.3 mmol) was added to the reaction mixture, then the color of the solution became bright reddish-brown. Purple crystals were obtained from this solution. The elemental analysis, and the visible and infrared absorption spectra confirm that these crystals are Na[V I I I (edta)(H 2 O)]. Thus, it was concluded that L-cysme can reduce vanadium(IV) to vanadium(III) in water in the presence of edta.