The formation constants of the species formed in the systems H+ + Mo(VI) + cysteine and H+ + cysteine have been determined in aqueous solution in the range 4 less than or equal to pH less than or equal to 8 at 25 degreesC and different ionic strengths ranging from (0.1 to 1.0) mol.dm(-3) (NaClO4), Using polarimetric, spectrophotometric, and potentiometric techniques. The composition of the complex was determined by the continuous variations method. It was shown that molybdenum(VI) forms a mononuclear 1:1 complex with cysteine of the type MoO3L2- at -log [H+] = 5.8. The dependence of protonation and the stability constants of cysteine and its complex on ionic strength are described by a Debye-Huckel type equation, and finally the results have been compared with data previously reported and interpreted.