Oxime ester compounds of the formula I, II, III, IV or V wherein Z is for example Z1 for is NO2, unsubstituted or substituted C7-C20 aroyl or unsubstituted or substituted C4-C20heteroaroyl; provided that at least one Z1 is other than NO2; Z2 is for example unsubstituted or substituted C7-C20aroyl; R1, R2, R3, R4, R5 and R6 for example are hydrogen, halogen, or unsubstituted or substituted C1-C20alkyl, unsubstituted or substituted C6-C20aryl, or unsubstituted or substituted C4-C20heteroaryl; R9, R10, R11, R12 and R13 for example are hydrogen, halogen, OR16, unsubstituted or substituted C1-C20alkyl; provided that R9 and R13 are neither hydrogen nor fluorine; R14 is for example unsubstituted or substituted C6-C20aryl or C3-C20heteroaryl Q is for example C6-C20arylene or C3-C20heteroarylene; Q1 is —C1-C20alkylene-CO—; Q2 is naphthoylene; Q3 is for example phenylene; L is for example O-alkylene-O—; R15 is for example hydrogen or C1-C20alkyl; R20 is for example hydrogen, or unsubstituted or substituted C1-C20alkyl; are effective photoinitiators.