Metal–nitrogen–carbon (MNC) catalysts represent a potential means of reducing cathode catalyst costs in low temperature fuel cell cathodes. Knowledge-based improvements have been hampered by the difficulty to deconvolute active site density and intrinsic turnover frequency. In the present work, MNC catalysts with a variety of secondary nitrogen precursors are addressed. CO chemisorption in combination with Mössbauer spectroscopy are utilized in order to unravel previously inaccessible relations between active site density, turnover frequency, and active site utilization. This analysis provides a more fundamental description and understanding of the origin of the catalytic reactivity; it also provides guidelines for further improvements. Secondary nitrogen precursors impact quantity, quality, dispersion, and utilization of active sites in distinct ways. Secondary nitrogen precursors with high nitrogen content and micropore etching capabilities are most effective in improving catalysts performance.