Among the most useful methods for the preparation of pure metal powders is the thermolysis of metal carbonyl complexes in hydrocarbon solutions. Zero-valent cobalt particles are obtained by the decomposition of Co-2(CO)(8). The reaction is primarily governed by diffusion, which is strongly dependent on the viscosity of the solution. In a solution containing polystyrene, the viscosity is directly proportional to the concentration of the polymer. To study the variation of the rate constants of this colloidal reaction as a function of the solution viscosity, we examined various polystyrene solutions of different molecular weights and concentrations. The reaction rate increases considerably at polymer content below and at the critical coil overlap concentration. Above this concentration, as the polymer coils become entangled and contracted, the decreased mobility of the molecules due to higher viscosity and lower diffusion rate lowers the reaction rate. The influence of the molecular weight of the polystyrene on the reaction kinetics had a "catalytic" effect on reaction rates and was most dramatic with a MW(avg) of similar to 120 000. We find that in the dilute polymer regime, below the PS coil overlap threshold, the polymer chains can provide the necessary support for the aggregation of the cobalt particles. Also, in the dilute regime, the mobility of the cobalt molecules is not hindered due to the low solution viscosity. However, there is evidently a critical molecular weight at which the "catalytic" effect of the polymer is at its maximum. These results will be discussed, and possible mechanisms for the polymer-enhanced colloid reactions will be offered.