Nonnoble metal catalysts for the electrochemical reduction of oxygen in acidic medium have been produced by adsorbing iron(II) acetate on 19 carbon supports. These materials were then pyrolyzed in an atmosphere containing NH3. The 19 carbon supports are (i) six as-received commercial supports (Printex XE-2, Norit SX Ultra, Ketjenblack EC-600JD, acetylene black, Vulcan XC-72R, and Black Pearls 2000), (ii) three as-received developmental supports (Lonza HS300 and Sid Richardson RC1 and RC2), (iii) the same nine previous supports prepyrolyzed at 900degreesC in an atmosphere containing NH3 to increase their N content, and (iv) a synthetic carbon made by pyrolyzing perylene tetracarboxylic dianhydride at 900degreesC in an atmosphere containing NH3. The goal of this study is to determine the effect of the carbon support on the catalytic activity of the catalysts. The specific surface area, the pore size distribution, the N and O contents, and the electrocatalytic activities of the 19 types of catalysts were measured. It was found that the activity of the catalysts varies greatly from one carbon support to another, but neither the specific surface area of the catalysts nor the distribution of their macro- or mesopores is a determining factor for the catalytic activity. The most important factor is the N content of the materials; the higher it is, the higher is the density of the catalytic sites on their surface and the better is the electrocatalyst. Carbon supports that are devoid of N, however, display some lower catalytic activity, which is attributed to an iron oxide. The latter catalytic site occurs also in the other N-containing catalysts. In these materials there are, therefore, three catalytic sites at work: an iron oxide site and two N-containing sites labeled FeN4/C and FeN2/C, with the last site being the most active for oxygen electroreduction.