Abstract: A keto arene-cyclopentadienyl iron complex salt of the formula:[(R.sup.2 CO)(R.sup.1).sub.m C.sub.6 H.sub.5--m Fe.sup.II Cp].sup.+1 A.sup.-1wherein R.sup.1 is an alkyl group, R.sup.2 is an alkyl group or phenyl group, m is an integer of 1 to 5, Cp is an .eta..sup.5 -cyclopentadienyl group and A.sup.-1 is a non-nucleophilic anion, e.g. pentamethylacetophenone-Fe(II)-Cp.sup.+ PF.sub.6.sup.-1 salt or pentamethylbenzophenone-Fe(II)-Cp.sup.+ PF.sub.6.sup.-1 salt, which is useful as a photoinitiator for cationically polymerizable compounds, especially for epoxy resins, and has an improved environmental safety and a high absorptivity, and which can be easily produced by a direct replacement reaction between dicarbonyl-.eta..sup.5 -(2,4-cyclopentadien-1-yl)iron(II) halide and an alkyl substituted keto arene to be replaced for the carbonyl ligands.