The photoinitiation efficiency of the fluorenone/triethylamine (TEA) system in the polymerization of methylmethacrylate (MMA) has been evaluated as a function of the monomer concentration, the amine concentration, and the polarity of the reaction medium. The polymerization proceeds readily in low polarity media (benzene/monomer), but it is negligible in more polar solvents (acetonitrile/monomer). The polymerization rate increases with the amine concentration up to 0.01 M TEA. Further increase in amine concentration produces a decrease in the polymerization rate. A similar behavior was observed for the fluorenone photoreduction yield and the yield of fluorenone derived radicals. All these processes are considered to involve the excited triplet, while quenching of the excited singlet by the amine decreases the rate of these processes. However, the decrease in photoinitiation efficiency observed at high amine concentration is larger than that expected from the singlet quenching extent, as estimated from the effect of the amine on the fluorescence yield under the same experimental conditions. This discrepancy indicates that other process(es) must contribute to the protection afforded by high amine concentrations. Quenching of the charge transfer intermediate by the amine is postulated as a competitive process that could explain the above mentioned effects.