Equilibria of the reaction of molybdenum(VI) with aspartic acid have been studied in aqueous solution of pH 5-6.5 using spectrophotometric and optical rotation methods at constant ionic strength (0.15 mol dm(-3) sodium perchlorate) and (25 +/- 0.1) degrees C. Our studies have shown that aspartic acid forms a mononuclear complex of 1:1 with Mo(VI) at pH 5.8, and the stability constant of this complexation has been determined.