A catalytic method has been developed for the determination of microgram amounts of mercury (II) based on its catalytic effect on the rate of a ligand substitution reaction. The reaction studied involved the substitution of cyanide in hexacyanoferrate (II) by resacetophenone benzoic acid hydrazone (2’, 4’- dihydroxy propiophenone benzoic acid hydrazone) and the reaction was monitored spectrophotometrically. Satisfactory results were obtained for the determination of mercury (II) in the range 0.1086-0.9774 µg/ml. The relative standard deviation was less than 2.1%. The method detection limit and limit of quantification was found to be 0.041 µg/ml and 0.083 µg/ml respectively (n=7). The correlation coefficient for the determination was found to be 0.9989. The effect of foreign ions on the determination was studied.