Methyl methacrylate (MMA) has been photopolymerized in methanol using the photoinitiation system formed by the dye thionine and triethanolamine as co-initiator. In the absence of amine no polymerization was observed, and at very high concentrations of the amine the polymerization rate decreased. The maximum polymerization rate was obtained with around 0.3 M amine. The individual rate constants for the reactions involved in the photoinitiation process have been determined by static fluorescence for the singlet state and flash photolysis for the triplet states. A mechanism is presented for the photoinitiation, from which polymerization rates could be calculated using the rate constants for each of the steps. These results were consistent with the measured experimental polymerization rates.