Because of its potential relevance to the Gratzel photoelectrochemical cell, we have investigated the oxidation of iodide by [Fe(bpy)(2)(CN)(2)](+) in acetonitrile solution at 25 degreesC. The reaction occurs on the stopped-flow time scale and is highly sensitive to catalysis by Cu2+, to the degree that, under typical conditions, the impurity levels of Cu2+ are sufficient to dominate the reaction. 2,2'-Bipyridine is a very effective inhibitor of the Cu2+ catalysis, and 1 muM Cu2+ can accelerate the rates by as much as a factor of 280, relative to the bpy-inhibited reaction.