The accurate potentials of Ph2I+ cation reduction, yielding in one step Ph• and PhI, are equal to −0.68, −0.61 and −0.67V vs. SCE, in MeCN, DMF and water, respectively, according to state-of-the-art quantum chemical calculations using the benchmark gas-phase energetics (the C–I bond homolytic dissociation energy in Ph2I+ and electron affinity of PhI•+) from the explicitly correlated coupled-cluster method (CCSD(T)-F12) together with the SMD solvation free energies. The alkyl (Me or t‐Bu) substitution effect on the reduction potential of Ph2I+ was estimated to be less than 50mV. It was demonstrated that employing the ubiquitously used erroneous E°(Ph2I+) value of −0.2V in the prediction of ΔG° for photoinduced electron transfer may lead to failed photoinitiated polymerisation experiments.