The photochemical and photopolymerisation properties of 1-fluoro-4-propoxythioxanthone (FPTX), 1-chloro-4-propoxythioxanthone (CPTX) and 1-bromo-4-propoxythioxanthone (BPTX) have been determined and compared with the corresponding activity of 2-propoxythioxanthone (ITX). Both CPTX and BPTX exhibit high photoinitiation activity when used alone and in conjunction with a tertiary amine cosynergist and benzophenone. The ITX and FPTX are active only when used with a tertiary amine cosynergist and/or benzophenone. Absorption and luminescence spectra indicate that the lowest excited states possess mixed ππ*/nπ* character with the BPTX exhibiting the highest phosphorescence quantum yield probably due to heavy-atom spin–orbit coupling by the Br atom. Photolysis/photoreduction quantum yields are higher in a reductive solvent such as propan-2-ol than in non-reductive acetonitrile for the three halogenated derivatives. They are enhanced further in the presence of a tertiary amine. These data contrast with that for ITX where photolysis/photoreduction is observed only in reductive propan-2-ol, in the absence and presence of a tertiary amine. Ketyl radical formation, with a wavelength maximum at 390 nm on microsecond flash photolysis, is similar for all the thioxanthones and is virtually insensitive to oxygen except for the BPTX derivative. The three halogenated derivatives give strong triplet–triplet absorptions on nanosecond laser flash photolysis in propan-2-ol and acetonitrile, similar to those for ITX. Quenching of the triplet state by a tertiary amine is found to increase with increasing amine concentration, with ITX being the most sensitive initiator. This quenching is consistent with their behaviour as photopolymerisation initiators. Bimolecular triplet quenching by naphthalene gives rate constants for ITX and FTPX which are similar to that of benzophenone while that for CPTX is an order of magnitude less. From these data we have determined that the molar absorption coefficients of the triplet–triplet absorptions for FPTX and CPTX are half that of ITX with all three being greater than that of benzophenone. The FPTX is also found to exhibit a significantly lower yield of intersystem crossing compared with CPTX and ITX. The latter two initiators exhibit rates of intersystem crossing which are almost comparable with that of benzophenone. Values of molar absorption coefficients and rates of intersystem crossing for BPTX could not be measured since the laser flash studies indicated the formation of a charge-transfer complex with triplet quenchers such as naphthalene. Solution photolysis studies indicated an additional reaction involving subsequent scission of the halogen–thioxanthone bond in the case of BPTX and CPTX but the FPTX gave no such reaction. Various mechanisms are discussed to account for the effective photoinitiation activity of BPTX and CPTX including their potential role as hybrid initiators.