Compounds of the formula (I) and (II) M1, M2 and M3 independently of one another are no bond, a direct bond, CO, O, S, SO, SO2 or NR14; provided that at least one of M1, M2 or M3 is a direct bond, CO, O, S, SO, SO2 or NR14; M4 is a direct bond, CR″3R″4, CS, O, S, SO, or SO2; Y is S or NR18; R1 for example is hydrogen, C3-C8cycloalkyl, phenyl or napthyl, both of which are optionally substituted; R2 for example is C1-C20alkyl; R″2 has one of the meanings given for R2; R3 and R4 are for example hydrogen, halogen, C1-C20alkyl; R′3, R′4, R″3 and R″4 independently of one another have one of the meanings given for R3 and R4; and R5 is for example hydrogen, halogen, C1-C20alkyl; provided that in the compounds of the formula (I) at least two oxime ester groups are present and provided that at least one specified substituent R2 or R″2 is present; exhibit an unexpectedly good performance in photopolymerization reactions.