The formation of platinum sols ([d] = 1.5-4 nm) in a methanol-water mixture by methanol reduction of ionic platinum in the presence of-steric stabilizer poly(vinylpyrrolidone) has been studied by transmission electron microscopy and UV-visible spectrophotometry. A high concentration of chloride ions present in the reaction mixture is demonstrated to promote monodispersity and near-spherical particle shape of the metal colloids, favoring smoother rounder surfaces; at a constant mass ratio of polymer to H2PtCl6 without addition of chloride ions, scaling the preparations up to higher concentration also yields less agglomerated sols. Only a proportion of the total reduction (monitored as pH) precedes the first appearance of the diagnostic sol absorbance; the disappearance of the PtCl62- UV extinction with time is in accord with the pH change. The shape of the visible extinction spectrum of the resulting platinum sol reflects the aggregation and annealing processes that occur as the metal particles are produced, and the annealing of irregular-shaped coalescents in sol is shown to be greatly accelerated by hydrothermal treatment.