Novel thioxanthonecarboxylic acids and thioxanthonecarboxylic acid derivatives of the formula I ##STR1## in which Y is --COOH, --CO-halogen, --CN or a carboxylic acid ester, thioester or amide group and X, Z and W are as defined in the patent claim, are described. The compounds of the formula I in which Y is other than --CO--halogen are suitable as sensitizers for photo-crosslinkable polymers or as initiators for photo-polymerization of ethylenically unsaturated compounds or for photo-chemical crosslinking of polyolefins. The acid halides of the formula I are starting materials for the preparation of the corresponding nitriles and carboxylic acid esters, thioesters and amides.