Speciation of iron(III) (c(Fe) = (0.5 - 40) x 10(-5) mol dm(-3)) and indirectly of iron(II), in aqueous solution of L-aspartic acid and aspartic acid - glycine mixture was investigated by square-wave voltammetry on a static mercury drop electrode. A reversible, one-electron reduction process of iron(III)-aspartate complexes with peak potential between +0.04 and -0.17V was recorded, depending on the solution pH (4.5-8.6) and concentration of aspartic acid (c(Asp) = 0.01 - 0.4 mol dm(-3)). Cumulative stability constants of iron(III)-aspartate complexes: FeAsp(+) (log K= 13.16 +/- 0.01), FeAspOH(0) (log beta = 20.76 +/- 0.02), FeAsp(OH)(2) - (log beta = 27.77 +/- 0.12) and Fe(Asp)(2) - (log beta =17.62 +/- 0.10); and iron(II)-aspartate complexes: FeAsp(0) (logK = 4.17 +/- 0.11) and Fe(Asp)(2)(2-) (log beta = 6.53 +/- 0.11), were determined at I-c = 0.5 mol dm(-3) and 25 +/- 1 degrees C. In the solution with mixture of amino acids, aspartic acid and glycine, a formation of mixed (Fe(III)-aspartate-glycine) complexes occurred. The reduction process was also shown to be reversible, so stability constants of mixed complexes (iron(III): FeAspGly(0) (log beta = 17.35 +/- 0.02), FeAspGly(OH)(-) (log beta = 23.87 +/- 0.33), and iron(II): FeAspGly(-) (log beta = 6.83 +/- 10.23)) were calculated, as well. (C) 2014 Elsevier Ltd. All rights reserved.