The first part of this program addresses the analytical techniques that can be used to characterise platinum on carbon supported electrocatalysts for use in hot phosphoric acid fuel cells. The uses of various adsorbing species are reviewed, and a choice was made to examine in more detail the adsorption of rhenate ions on the platinum crystalline surface, to diagnose the platinum surface area within the operating fuel cell. This information is needed to understand how that platinum surface is modified with time in the operating environment. The second part of this program develops an advanced computer model for the porous electrode structure to include all of the resistive and diffusive losses, both in the electrode structure itself, and within the carbon backing paper on which the electrocatalyst layer is supported. The two respective models were combined to show the influence of catalyst utilization between the electrodes to predict the optimum loading of the platinum catalyst within the fuel cell assembly so that the fuel cell performance is maximised. 72 refs., 59 figs., 22 tabs.