Abstract: Michler's ketone analogs and salts thereof, which are photosensitizers in water-soluble photoinitiator systems used to induce polymerization in free-radically-curable, ethylenically-unsaturated materials, are disclosed. The Michler's ketone analogs have the formula: ##STR1## wherein each R is an alkyl group of 1 to 8 carbon atoms,R.sup.1 is an alkylene group having 1 to 8 carbon atoms,Z is R.sup.1 COOH or R.sup.1 H, andn is an integer having a value of 1 or 2.The photoinitiator systems are useful in imaging systems, and additionally contain a free radical initiator compound selected from iodonium salts, biimidazoles, trialkylphosphites, alkyl peroxides, benzyl halides, alkyl nitrates, and the benzophenones other than (a).