Acetals are important molecules with versatile reactivity and uses. For the first time a simple photo-catalysed facile synthesis of formaldehyde acetals is documented herein upon the reaction of alcohols with dimethyl sulfoxide under very mild conditions in the presence of air. The reactions require only 1 mol% of Ru^II(bpy)₃Cl₂ photocatalyst under blue LED irradiation (λ = 445 nm) to give good to excellent yields of the corresponding acetal products. Here DMSO acts as a “methylene” source.