Mercury (II) is able to quench the fluorescence of Rhodamine B in the presence of potassium iodide. The quenching was affected by pH of the solution and the concentration of potassium iodide, but not markedly by several anions. The degree of quenching was closely correlated with mercury (II) concentrations at the fixed concentrations of the dye and potassium iodide, so that mercury (II) can be fluorometrically determined in this method. Cystline and egg albumin protected the quenching by mercury (II) in the presence of potassium iodide. The fluorescence of Rhodamine B was not quenched by organic mercury (II) compounds in the presence of potassium iodide. Fluorescence of the dye was also quenched by some metal ions, such as bismuth (III), thallium (III), palladium (II) and platinum (IV) in the presence of potassium iodide.