Diaryliodonium tetrakis (pentafluorophenyl) berate salts generate a higher reactivity than any other known diaryliodonium salt. The photochemical properties of diaryliodonium tetrakis (pentafluorophenyl) borate salts were compared to those of the diaryliodonium hexafluoroantimonate salt. The results show that these new salts are the most reactive photoinitiators in this family. In addition, diaryliodonium tetrakis (pentafluorophenyl) berate salts are soluble in low polarity media, such as epoxy silicone oils, which are rich in epoxy groups and insensitive to humidity. These salts have the advantage not to contain a heavy metal (such as antimony). The new properties generated by the use of the tetrakis (pentafluorophenyl) berate anion make the future of the cationic photopolymerization promising.