The influence of loading on the activity and stability of heat-treated carbon-supported cobalt phthalocyanine electrocatalysts for oxygen reduction was studied. The Co loading on the carbon support was varied from 0 to 8 weight percent (w/o), while heat-treatment temperatures ranging from 500 to 1100 C were studied. There is an optimum value of the Co loading on the carbon support (about 3.5 w/o) over which the catalytic activity of the Co on carbon catalyst decreases. This trend is observed in both half-cell (rotating disk electrode measurements) and H{sub 2}/O{sub 2} fuel cell measurements. The optimum value of the Co loading on the carbon support is independent of the heat-treatment temperature. An explanation is proposed for this observation. The authors have also performed short-term (1.5 to 18 h) life tests with the 3.5 w/o Co on carbon-support materials heat-treated at various temperatures. It is shown that the current density lost after 18 h of operation is a function of the heat-treatment temperature being lower the higher the temperature. The observed increase in the stability of the material with the heat-treatment temperature is best explained by considering the protective role played by the graphitic-like layer surrounding the Co metal particles.