The photopolymerization of methyl methacrylate initiated by substituted thioxanthones in the presence of an amine has been investigated. The polymerization rates in acetonitrile were measured at several amine concentrations. The dependence of the polymerization rates with the amine concentration and the photoinitiation efficiency at a given amine concentration are highly dependent on the ketone structure. The rare constants for the quenching of thioxanthones excited states by the monomer and the amine were measured by fluorescence and laser hash photolysis, under the polymerization conditions. These data show that the reactivity of the ketone is also highly dependent on the ketone structure. The values of the quenching rate constants show that the initiation efficiency is controlled by the competition between the quenching of the excited states by the monomer and also the amine. From the latter a mechanism is proposed to simulate the dependence of the polymerization rates with amine concentration.