The complexation of Mo(VI) with dopamine has been investigated by spectrophotometric measurements in a mixed solvent system at an ionic strength of 0.2 mol.dm(-3) sodium chloride, employed [(15, 25, or 35 +/- 0.1) degrees C] at pH ranges of similar to 4 to similar to 7 with a high ratio of ligand to metal. The effect of solvent systems on protonation and complexation are discussed. Linear relationships are observed by plotting log K versus 1/D, where K and D are the stability and dielectric constants, respectively.