The kinetics and mechanism of the reaction of aqueous Hg(II) with methyl iodide have been investigated. The overall reaction is best described as Hg(II)-assisted hydrolysis, resulting in quantitative formation of methanol and, in the presence of excess methyl iodide, ultimately, Hgl(2) via the intermediate Hgl(+). The kinetics are biexponential when methyl iodide is in excess. At 25 degrees C, the acceleration provided by Hg2+ is 7.5 times greater than that caused by Hgl(+), while assistance of hydrolysis was not observed for Hgl(2). Thus, the reactions are not catalytic in Hg(II). The kinetics are consistent with an S(N)2-M+ mechanism involving electrophilic attack at iodide. As expected, methylation of mercury is not a reaction pathway; traces of methylmercury(II) are artifacts of the extraction/preconcentration procedure used for methylmercury analysis.