A photoinitiator mixture comprising the components (a) a compound of the formula (I) wherein Ar1 and Ar2 independently of each other are  or naphthyl which is unsubstituted or substituted one or more times by R1, R2, R3 or R′; R1 and R3 in-dependently of each other are C1-C4alkyl, C1-C4alkoxy or halogen; R2 is hydrogen, C1-C4alkyl, halogen, C1-C4alkoxy or C2-C20alkoxy which is interrupted by one or more O; Q is C1-C4alkylene; R4 is methyl or ethyl; R′ and R″ independently of each other are hydrogen or PG-Y—R′″—X—; PG is a polymerizable group or methyl or ethyl; Y is a direct bond, O or S; X is a direct bond, O or S; R′″ is a direct bond, C1-C20alkylene or C2-C20alkylene which is interrupted by one or more O; (b) one or more compounds of the formula (II) wherein Ar1, Ar2 and Q are as defined above, and R5 is for example C3-C30alkyl which is unsubstituted or substituted and (c) optionally a compound of the formula (III) R5—OH  (III), wherein R5 is as defined above; provides a liquid photointiator.