The main focus of the present thesis is on corrosion and prevention of corrosion of platinum particles supported on carbon. This is important for instance in connection with start up and shutdown of fuel cells. The degradation mechanism of platinum particles supported on carbon has been characterized during oxygen reduction reaction (ORR) condition using identical location (IL) transmission electron microscopy (TEM). A TEM grid was used as the working electrode in an electrochemical setup allowing a direct correlation between the electrochemical response and the TEM analysis. The main results of the corrosion was a decrease in the particle size, some particles disappeared and other particles sintered. The TEM observations of the platinum particles provide evidence that dissolution of platinum particles is one of the main causes of degradation of platinum particles supported on carbon under ORR conditions. In the present work the corrosion stability of three commercial catalysts have been investigated. Even though they have similar specifications they have different corrosion stabilities. From an industrial point of view it is interesting that the catalyst supplied by company 2 is much more corrosion stable and has a higher utilization of the platinum surface area than the other catalysts investigated. The experiments preformed revealed that platinum particles can be stabilized against corrosion by adding gold to the platinum particles. In the present work electrocatalysts with one third, two thirds and one monolayer of gold on platinum supported on carbon were synthesized by an inverse micelle method. The results obtained appear independent of the gold coverage. It has been shown that the electrochemical active surface areas of the platinum and platinum gold particles synthesized by the inverse micelle method were very low compared to the theoretical calculated surface areas. In the present work it was attempted to synthesize 5 nm platinum particles supported on carbon by the polyol method. The performed experiments showed that a gaussian particle size distribution of platinum particles is obtained when the carbon dispersion is added after the reduction of the platinum particles. If the reduction of the platinum particles are preformed before adding the carbon dispersion a bi normal size distribution of platinum particles is obtained. The level of water in the polyol method does not appear to have the effect on the platinum particle size as reported in the literature.