A~tract--The photochemistry and photopolymerization activity of a novel initiator, 3,4-dimethyl-2-(3-N,N-dimethyl)-n-propoxythioxanthone, has been compared with that of the non-aminated molecule, 3,4-dimethyl-2-n-propoxythioxanthone. Using photodilatometry, the rate of photopolymerization of n-butyl methacrylate is significantly greater for the aminated derivative in the absence or in the presence of a tertiary amine co-synergist. In the former case, luminescence spectroscopy indicates that lower fluorescence quantum yields are consistent with higher rates of intersystem crossing giving correspondingly higher phosphorescence quantum yields. Here the solvent must be competing with any intra-or intermolecular hydrogen bonding between the amine functionality and carbonyl groups. On conventional micro-second flash photolysis, strong transient absorptions are observed due to both the ketyl radical and radical anion species in a non-reductive polar solvent acetonitrile. In this solvent system, intra-and intermolecular exciplex formation is evidently involved in radical formation. Using second order derivative absorption spectroscopy, the thioxanthone chromophore is identified in the structure of poly(methyl methacrylate) prepared using the aminated derivative so confirming that the alkylamino radical is involved in the initiation of polymerization.