One of the most important concerns in catalyst synthesis is metal particle size. In heterogeneous catalysis, chemical reactions of the gas phase occur with the metal atoms at the surface of the particles. A high level of dispersion is critical to maximize the amount of surface metal atoms. After impregnation of the support with the metal precursor, the pretreatment conditions of the catalyst play a crucial role in determining the final metal particle size. This correlation has been indirectly investigated by various studies, but hasn’t been systematically examined. This research deals with the influence that temperature and heating rate, during reduction in hydrogen gas, have on platinum and palladium particle size on various supports, both low PZC (oxidized carbon, TiO2, SiO2) and high PZC (carbon, Al2O3). The influence of humidity during reduction is also studied for the Pt catalysts. The shape of the particle size distribution offers valuable information to determine if a particular sintering mechanism, Ostwald Ripening (OR) or Particle Migration and Coalescence (PMC), has taken place. For this study, the catalysts are synthesized by dry impregnation (DI) and strong electrostatic adsorption (SEA). Particle size analyses were done by XRD and STEM imaging. Even though particle sizes widely vary between supports, the metal used, and the preparation method, general results show a range from 0.9 to 15 nm, where lower heating rates and lower reduction temperatures favor the formation of smaller particles. The presence of humidity during reduction was found to cause extensive sintering. The particle size distributions obtained from STEM imaging reveal PMC is the predominant sintering mechanism at work although further evidence suggests OR could be happening as well for some systems.