Tetrafluoroethylene, trifluoroethylene, and chlorotrifluoroethylene react on irradiation with trans-Os(CO),-[(MeO),P], to afford the complexes Os[fluoro-olefin] (CO),[(MeO),P],, where the phosphite ligands have a relative cis-configuration. In contrast, trans-Os(CO),(PhPMe,), reacts with trifluoroethylene to give Os[C,F,H] -(CO),( PhPMe,), where the phosphine groups have a relative trans-configuration. The corresponding reactions with hexafluoroacetone gives OsC (CF,) , O (CO),( PhPMe,),. Reaction of Os( CF, C, CF, ) (CO),[ (MeO),P], with HCI gives OsCI[C(CF,) = C(CF,)H](CO),[(MeO),P],.The reaction of iodine with trans-Os(CO),(PhPMe,), is described.