Carbon-supported Ag (Ag/C) catalysts with four different metal loadings were prepared by a citrate-protecting method. Oxygen reduction reaction (ORR) activities on these carbon-supported Ag-nanocatalysts (Ag/C) in alkaline solutions were studied. Four major findings are reported in this paper: (1) Test results indicate that the Ag/C catalysts promote predominately a four-electron pathway for ORR on electrodes over the swept potentials from 0.2 to −0.8 V vs Hg/HgO in O₂-saturated 0.1 M NaOH solutions. (2) A novel marker for predicting ORR activities on the Ag/C catalysts based on the cyclic voltammetry characteristic curves has been identified: the ORR activities have a strong correlation with the intensity of the anodic peak at the potential of 0.230 V vs Hg/HgO in Ar-saturated 0.1 M NaOH solutions. (3) As the metal loading on carbon particles increases from 10 to 60 wt %, the peak intensities increase linearly, and the ORR onset potentials shift positively with maximum shift of 62 mV for 60% Ag on carbon support. (4) A hitherto unnoticed poisoning effect has been discovered: silicate has a significant poisoning effect on the ORR activities of the Ag/C catalysts.