Mononuclear bis(thienylselenolato)bis(triphenylphosphine)platinum [Pt(SeTh) 2 (PPh 3 ) 2 ] (Th=2-thienyl, C 4 H 3 S) has been prepared by the treatment of cis-[PtCl 2 (PPh 3 ) 2 ] with NaSeTh. The 3 1 P-NMR spectroscopic information indicates that cis-[Pt(SeTh) 2 (PPh 3 ) 2 ] is initially formed in the reaction. Upon prolonged standing in solution it isomerizes to trans-[Pt(SeTh) 2 (PPh 3 ) 2 ]. The reaction of cis-[PtCl 2 (PPh 3 ) 2 ] with LiSeFu (Fu=2-furyl, C 4 H 3 O) affords immediately a mixture of cis- and trans-isomers of [Pt(SeFu) 2 (PPh 3 ) 2 ] with the relative amount of the trans-isomer increasing with time. The recrystallization of the two reaction mixtures yielded cis,anti- and trans,syn-isomers of [Pt(SeTh) 2 (PPh 3 ) 2 ] as well as cis,syn- and trans,anti-isomers of [Pt(SeFu) 2 (PPh 3 ) 2 ]. Their structures were compared with those of cis,anti- and trans,anti-isomers of [Pt(SePh) 2 (PPh 3 ) 2 ]. The geometries and relative stabilities of all isomers of [Pt(SeTh) 2 (PH 3 ) 2 ], [Pt(SeFu) 2 (PH 3 ) 2 ] and [Pt(SePh) 2 (PH 3 ) 2 ] were studied by the use of ab initio molecular orbital techniques in order to model the structures and isomerization of the observed mononuclear selenolato complexes.