The objective of this study was to produce well-defined palladium and platinum nanoparticles and investigate the effect of nanoparticle size and synthetic methodologies in the catalytic activities. Two types of template, hydroxyl terminated poly(amidoamine) dendrimer template and reverse microemulsions using sodium dietylhexylsulfosuccinate (AOT) surfactant/isooctane/water system, were utilized to produce Pd and Pt nanoparticles. The average diameter (d) of Pd and Pt nanoparticles were 1.9-2.5 nm by the dendrimer-template method, 3.5-5.1 nm by the reverse microemulsions. Overall, smaller sizes with narrower size distributions were obtained by the dendrimer template method. The reduction of 4-nitrophenol (4-Nip) was selected as a model reaction to fulfill the objective of this study. The surface normalized rate constant (k(1)), which takes into account the size of nanoparticle, was utilized to compare the catalytic activities of various catalytic systems in literature as well as in this study. The effect of nanoparticle size and synthetic methodologies on the catalytic activities will be discussed based on The results indicate that Pd based nanoparticles presented higher catalytic activities as compared to Pt based nanoparticles and the nanoparticle produced by the dendrimer template showed superior activities to that of the reverse microemulsions for the reduction of 4-Nip. (C) 2015 Elsevier B.V. All rights reserved.