Compounds of the Formula (I) wherein x is an integer from 1-4; p is an integer from 1-3; q is an integer from 0-3; Ar is phenyl, naphthyl, anthryl or phenanthryl each of which optionally is substituted by one or more Cl, CN, OR.sub.5, C.sub.3-C.sub.5alkenyl or C.sub.1-C.sub.6alkyl which optionally is substituted by one or more OR.sub.6, COOR or halogen; R.sub.1 if x is 1, is OR.sub.S, O--X.sup.+, NR.sub.8R.sub.9, C.sub.1-C.sub.20alkyl optionally substituted by one or more COOR.sub.10, or is C.sub.2-C.sub.20alkyl interrupted by one ore more O, or is C.sub.2-C.sub.5alkenyl or phenyl-C.sub.1-C.sub.4alkyl; R.sub.1 if x is 2, is for example C.sub.1-C.sub.20alkylene; R.sub.1 if x is 3, is for example a tri-valent radical; R.sub.1if x is 4, is for example a tetravalent radical; R.sub.2and R.sub.3are hydrogen or C.sub.1-C.sub.8alkyl, or R.sub.2and R.sub.3 together are O, C.sub.1-C.sub.3alkylene or CH.dbd.CH; R.sub.4 is C.sub.1-C.sub.4alkyl; R.sub.5, R.sub.6, R.sub.7, R.sub.8, R.sub.9 and R.sub.10 are for example hydrogen or C.sub.1-C.sub.4alkyl; and X is a x-valent cationic counter ion; are in particular suitable as photoinitiators for the curing with UV-A light (320-450 nm). ##STR00001##