Platinum and ruthenium nanoparticles (Pnp, Rnp) are found to catalyse the dye formation reaction of aniline (AN) with 4-aminoantipyrine (AAP) in an acidic aqueous medium. The kinetic parameters of the reaction were studied by UV-vis spectroscopy. Pseudo first order kinetics and the total order equal to 2.0 are detected in both Pnp and Rnp systems. In the presence and absence of microheterogeneous media, the rate and order of the antipyrilquinoneimine dye formation depends on the nature of the metal nanoparticles, pH, ionic strength and different microheterogeneous media such as sodium dodecylsulfate (SDS), cetyltrimethylammonium bromide (CTAB) and tritonX-100 (TX-100). In the acidic medium, the catalytic effect of Rnp is found to be greater than that of Pnp. In the microheterogeneous medium, the optimized reaction rate of product formation follows the order SDS > TX-100 > CTAB. A possible reaction pathway has been proposed involving the activation of AN and AAP by metal nanoparticles and concomitant reactions of free radicals. Transmission electron microscopy (TEM) results show that the particle sizes are 12.0 nm and 7.0 nm, respectively, for Pnp and Rnp in the aqueous medium. (c) 2006 Published by Elsevier B.V.