Oxalic acid monothioester (OAM), an easily accessible and storable reagent, was reported herein as a thioester synthetic equivalent for palladium-catalyzed decarboxylative thiocarbonylation of organohalides and hydrothiocarbonylation of unsaturated carbon-carbon bonds at room temperature with high chemo- and regioselectivity. The reaction is applicable to the synthesis of cysteine-derived thioesters, thus allowing chemical modification of cysteine-containing peptides. Decarboxylation of OAM proceeds through oxidative addition of Pd(0) to the acyl-S bond, which accounts for the very mild reaction conditions.