The effect of surfactant type and temperature on the kinetics of the formation of platinum nanoparticles in water-in-oil microemulsions by chemical reduction of PtCl 2- 6 were examined with timeresolved UV-vis absorption spectroscopy. The surfactants used were poly(ethylene glycol)monododecyl ethers (C 12 E 4 , C 12 E 5 , C 12 E 6 ), sodium bis(2-ethylhexyl)sulphosuccinate (AOT), and mixtures of the alcohol ethoxylates and AOT. The oil domain was n-heptane. The microemulsion droplet size was measured by a dynamic light scattering technique (photon correlation spectroscopy) and the final platinum particle size was determined by transmission electron microscopy. The reaction rate for platinum particle formation was approximately the same in microemulsions based on either of the alcohol ethoxylates but considerably lower for microemulsions based on AOT. In microemulsions based on mixtures of an alcohol ethoxylate and AOT the reaction rate was similar to that obtained when alcohol ethoxylate was the sole surfactant. The reaction was observed to be particularly rapid in microemulsions based on combinations of AOT and C 12 E 5 or C 12 E 6 , and the rate was relatively independent of the ratio of the nonionic and anionic surfactants. The reaction was found to be of first order for platinum nanoparticles formed in alcohol ethoxylate-, AOT-C 12 E 5 -, and AOT-C 12 E 6based microemulsions, whereas in microemulsions with AOT and AOT-C 12 E 4 the reaction rate seemed to be of higher reaction order. The platinum particles were found to be less than 5 nm in average diameter, which was consistent with the microemulsion droplet size.