A kinetic method is described for the determination of mercury(II), based on its catalytic effect on the ligand substitution reaction between hexacynoferrate(II) and Nitroso-R-Salt. The reaction is followed spectrophotometrically by measuring the absorbance at fixed times at 720 nm, the lambda(max) of green coloured complex, [Fe(CN)(5)NRS](3-) formed during the course of reaction under optimum reaction conditions viz, temp = 25.0 +/- 0.1 degrees C, pH = 6.5 +/- 0.02 and I = 0.1 M(KNO3) The method is simple, sensitive, selective, rapid and allows the determination of as little as 10(-6) M of mercury with good accuracy and precision. The detection limit is computed by standard procedure and found to be 1.6 x 10(-7) M of [Hg2+]. The effects of few interfering and non-interfering cations and anions on mercury(II) determination have also been reported with relative standard deviations.