Addition of appropriate amounts of Me2S unusually accelerated the hydrosilylation of aldehydes catalyzed by [Ru-3(CO)(7)(Acy)] (1, Acy: mu(3)-eta(2),eta(3),eta(5)-acenaphthylene). The reduction of ketones was completely suppressed under the reaction conditions. The highly selective reduction of aldehydes in the presence of ketones was achieved via a nonconventional mechanism.