Abstract Mononuclear bis(thienylselenolato)bis(triphenylphosphine)platinum [Pt(SeTh)2(PPh3)2] (Th=2-thienyl, C4H3S) has been prepared by the treatment of cis-[PtCl2(PPh3)2] with NaSeTh. The 31P-NMR spectroscopic information indicates that cis-[Pt(SeTh)2(PPh3)2] is initially formed in the reaction. Upon prolonged standing in solution it isomerizes to trans-[Pt(SeTh)2(PPh3)2]. The reaction of cis-[PtCl2(PPh3)2] with LiSeFu (Fu=2-furyl, C4H3O) affords immediately a mixture of cis- and trans-isomers of [Pt(SeFu)2(PPh3)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(PPh3)2] as well as cis,syn- and trans,anti-isomers of [Pt(SeFu)2(PPh3)2]. Their structures were compared with those of cis,anti- and trans,anti-isomers of [Pt(SePh)2(PPh3)2]. The geometries and relative stabilities of all isomers of [Pt(SeTh)2(PH3)2], [Pt(SeFu)2(PH3)2] and [Pt(SePh)2(PH3)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.