Glutaredoxins belong to the thioredoxin superfamily of structurally similar thiol-disulfide oxidoreductases catalyzing thiol-disulfide exchange reactions via reversible oxidation of two active-site cysteine residues separated by two amino acids (CX1X2C). Standard state redox potential (E degrees ') values for glutaredoxins are presently unknown, and use of glutathione/glutathione disulfide (GSH/GSSG) redox buffers for determining E degrees ' resulted in variable levels of GSH-mixed disulfides. To overcome this complication, we have used reverse-phase high performance liquid chromatography to separate and quantify the oxidized and reduced forms present in the thiol-disulfide exchange reaction at equilibrium after mixing one oxidized and one reduced protein. This allowed for direct and quantitative pair-wise comparisons of the reducing capacities of the proteins and mutant forms. Equilibrium constants from pair-wise reaction with thioredoxin or its P34H mutant, which have accurately determined E degrees ' values from their redox equilibrium with NADPH catalyzed by thioredoxin reductase, allowed for transformation into standard state values. Using this new procedure, the standard state redox potentials for the Escherichia coli glutaredoxins 1 and 3, which contain identical active site sequences CPYC, were found to be E degrees ' = -233 and -198 mV, respectively. These values were confirmed independently by using the thermodynamic linkage between the stability of the disulfide bond and the stability of the protein to denaturation. Comparison of calculated E degrees ' values from a number of proteins ranging from -270 mV for E. coli Trx to -124 mV for DsbA obtained using this method with those determined using glutathione redox buffers provides independent confirmation of the standard state redox potential of glutathione as -240 mV. Determining redox potentials through direct protein-protein equilibria is of general interest as it overcomes errors in determining redox potentials calculated from large equilibrium constants with the strongly reducing NADPH or by accumulating mixed disulfides with GSH.