Polymer or oligomer ionic compound made of a polycationic part (A.sup.+).sub.p comprising a plurality of onium units and a sufficient number of anions X.sup.- to provide electrical neutrality to the compound, characterized in thatthe onium units are selected from the group consisting of biaryliodonium, arylsulfonium, arylacylsulfonium, diazonium, organometallic cations comprising a transition metal which is complexed by at least one unsaturated cycle comprising 4 to 12 carbon atoms;X.sup.- is an imide anion [R.sub.F SO.sub.2 NSO.sub.2 R'.sub.F ].sup.- or a methylide anion [R.sub.F SO.sub.2 C(Q)SO.sub.2 R'.sub.F ].sup.- in which:1) Q represents:H--, Cl--, F--, Br--or CN--;an alkyl radical having 1 to 30 carbon atoms;an aryl, alkylaryl or arylakyl radical having 6 to 30 carbon atoms;a group R".sub.F or a group R".sub.F SO.sub.2 ;2) R.sub.F and R'.sub.F, as well as R".sub.F possibly when X.sup.- is a methylide anion, are independently selected from the group consisting of fluorine, perhaloalkyl groups having 1 to 30 carbon atoms, (perhaloalkyl)alkyloxy groups, perhalogenated cycloaliphatic groups having 3 to 30 carbon atoms possibly containing hetero atoms selected from O and N, and/or possibly comprising at least one perhaloalkyl chain, the perhalogenated aryl having 6 to 30 carbon atoms; or3) R.sub.F and R'.sub.F together form a bivalent radical constituting a cycle respectively with group --SO.sub.2 --N--SO.sub.2 -- or with group --SO.sub.2 --C(Q)--SO.sub.2 --, or when X.sup.- is a methylide anion, R".sub.F constitutes with one of the radicals R.sub.F or R'.sub.F a bivalent radical constituting a cycle respectively with group --SO.sub.2 --C--SO.sub.2 -- or group --SO.sub.2 --C--, said bivalent radical being selected from perfluorinated alkylene radicals having 2 to 12 carbon atoms, the third radical which is possibly present being selected from the monovalent radicals mentioned above in 2);4) p represents the number of onium units.These compounds are useful as photoinitiators for cationic polymerization or cross-linking of monomers or for the modification of polymers for example when they are used as photoresists. The preparation of these compounds is also disclosed.