A family of complexes of general formula [Pt(h 2 -Ol E ){(h 4 -CH 2 CHSiMe 2 ) 2 O}], where Ol E is a di-, tri-or tetrasubstituted electron-deficient olefin such as fumaronitrile, diethylfumarate, tetracyanoethylene, ethylenetetracarboxylate, a naphthoquinone or dihydronaphthoquinone, were synthesised from the Karstedt catalyst solution containing [Pt 2 (CH 2 CHSiMe 2 ) 2 O) 3 ] (1). The structures of these complexes were examined by 1 H, 13 C and 195 Pt NMR spectroscopy. The X-ray crystallographic structure of [Pt(h 2 -MeNQ){(h 4 -CH 2 CHSiMe 2 ) 2 O}] (MeNQ methylnaphthoquinone) was determined; the MeNQ ligand is bound to Pt by a h 2 -olefin interaction. Certain of these complexes, particularly the naphthoquinone and dihydronaphthoquinone derivatives, were found to catalyse very efficiently the hydrosilylation of a variety of olefins. Tests in the presence of Hg, dibenzo[a,e]cycloocta-tetraene or molecular oxygen indicated that the catalytic process was homogeneous in nature. The addition of excess Ol E to the catalyst solutions greatly extended the lifetime and productivity of these catalysts, which were more efficient both in rate and overall product yield than the original Karstedt catalyst solution. Spectroscopic studies lead us to propose that the stabilising ligand Ol E remains bound to Pt throughout the catalytic cycle; this results in increased stability and high catalytic activity.