Arsenic is metabolized by living systems using oxidation-reduction and methylation reactions, and reduced glutathione (GSH) has been shown to be important in that metabolism. In this study, the solution reactions between GSH and arsenate, arsenite, and their methylated metabolites, monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA), were characterized using 1H and 13C NMR under a nitrogen atmosphere. Binding to GSH through the thiol group was primarily followed by shifts in the carbon atom bonded to the sulfhydryl group of the cysteinyl residue, i.e., the CH2 carbon atom and the protons bonded to it. The methylated metabolites also showed shifts in the methyl groups attached to the arsenic atom after reaction with GSH. Sodium arsenite, As(III), bound to GSH to form an As(SG)3 complex in solution as indicated by NMR spectra. The identity of the complex was confirmed by FAB-MS after isolation of the compound. Mixtures of sodium arsenate, As(V), and GSH showed that arsenate oxidized GSH in D2O solutions at pH 7 to form oxidized glutathione (GSSG). When the molar ratio of As:GSH exceeded 1:2, evidence for the formation of As(SG)3 was observed. MMA and DMA are both As(V) species, and mixtures with GSH showed oxidation to GSSG initially followed by formation of CH3.As(SG)2 and (CH3)2.As.SG, respectively. The effects of GSH on arsenic metabolism may result from direct reactions between the two compounds.