The sensitized-photoreactions of diphenyliodonium salt and four of its technically relevant derivatives bearing lipophilic substituents were investigated by pseudo-steady-state and time-resolved CIDNP experiments. Singlet (naphthalene, diphenylanthracene, dimethylanthracene) and triplet (xanthone, thioxanthone, Michler's ketone) sensitizers were used in a variety of solvents (acetonitrile, dimethyl sulfoxide, dimethylformamide, methanol, chloroform, tetrahydrofurane, diethylene glycol dimethyl ether, dioxane, toluene). Under all conditions, the primary step leading to CIDNP was found to be electron transfer from the excited sensitizer *Sens to the onium cation On(+). All spin-polarization effects could be explained consistently within the framework of radical pair theory (S-T-0-type CIDNP). Pair substitution, i.e., the transformation of the primary radical pairs RP I (On(.)Sens(.+)) into secondary pairs RP 2 (Ph(.)Sens(.+), where Ph. is the phenyl or an aryl radical) plays a key role for the CIDNP effects and even leads to a field dependence of the polarization phases for the system diphenyliodonium cation/naphthalene in dioxane. Decreasing solvent polarity causes an increase of the rate RP 1 --> RP 2. The introduction of a long lipophilic side chain into the onium salt has the same effect, presumably owing to self-solvation.