Attempts to determine the stability constants for the methylmercury phosphato and diphosphato complexes at 25C in an aqueous media and with an ionic strength of 0.10 M (with KNO3) through the use of electromotive force measurement and half-wave potentials from the dropping mercury electrode are described. A glass-nitrate selective ion electrode combination yielded reproducible and meaningful values when a solution of the ligand and methylmercury ion were titrated with standard nitric acid. Calculations of stability constants proved to be impossible since the quoted constants for the hydrolysis of the methylmercury cation are apparently in error, and corrective efforts are outlined. Reduction waves of the methylmercury chloride and hydroxide species in solution are presented and explained by postulated mechanisms involving free radical intermediates.