Abstract: The invention provides novel thioxanthone photoinitiators having the formula: ##STR1## wherein R.sub.1 alkyl, alkoxy, alkylthio, halogeno, nitro, amino, alkyl-amino, di-alkyl-amino, hydroxy alkyl-amino, alkanoylamino, benzoylamino, N-alkanoyl-N-benzoyl-amino, sulphonamido, or acetyl, R.sub.2 is alkylene, A is --COOH, --SO.sub.3 H, --OSO.sub.3 H, or --O--CO--X--COOH (where X is such that HO--CO--X--COOH is a di- or tri-carboxylic acid of up to 8 carbon atoms), n is 0, 1, or 2 and m is 1 or 2, provided that when A is --COOH, m is 2, the aforesaid alkyl, alkoxy, alkanoyl, and alkylene residues containing up to 4 carbon atoms each, as the free acid or as a salt thereof. The photoinitiators may advantageously be used in water-based photopolymerizable compositions, e.g. for use in screen stencil production.