A study of the reaction of thioanisole with singlet oxygen in different ionic liquid-acetonitrile binary mixtures has shown that ILs are able to accelerate the thioanisole sulfoxidation when used as additives. With imidazolium ILs, the maximum efficiency is reached at x(IL) ∼ 0.1-0.2, whereas for the pyrrolidinium IL a plateau is reached. These results are discussed in terms of the ILs' tendency to form ionic aggregates and of differences in sulfoxidation reaction mechanism.