The reaction mechanism of the chalcogen atom replacement of phosphine chalcogenides has been revealed to be dissociative from a kinetic investigation for the reaction of phosphine selenide with sulfur. The dissociation is enthalpically promoted by the catalysis of Pd-0. For a bidentate phosphine chalcogenide, the intermediate phosphine was separated as the Pd-II complex by the catalysis and oxidative addition of Pd-0 complex. The novel catalytic replacement and dissociation of chalcogen atom are applicable to regeneration of phosphines from their oxides via the phosphine sulfides.