Photoreduction of dialkyl sulfoxides (R2SO) by an acid-stable NADH analogue, 10-methyl-9,10-dihydroacridine (AcrH2), proceeds in the presence of perchloric acid in acetonitrile containing H2O (0.50 M) to yield 10-methylacridinium ion (AcrH+) and the corresponding dialkyl sulfides (R2S). No photoreduction of R2SO by AcrH2 has occurred in the absence of perchloric acid. In the presence of perchloric acid the protonation of dialkyl sulfoxides occurs to increase the oxidizing ability of the sulfoxides significantly, when the singlet excited state (1AcrH2*) is quenched efficiently by the protonated sulfoxides (R2SOH+). The 1AcrH2* is also quenched by HClO4. The dependence of quantum yields on [R2SOH+] indicates that the photoreduction of R2SO by AcrH2 proceeds via electron transfer from 1AcrH2* to R2SOH+, followed by the hydrogen transfer from AcrH2•+ to R2SOH•, accompanied by the concomitant dehydration to yield AcrH+ and R2S.