A thermally stable polysiloxane composition is a cross-linked product of a Part A composition, of a Part B composition and a catalyst, the Part A composition being a polysiloxane having 60 to 75 percent by weight phenyl groups, the Part B composition being a polysiloxane having 50 to 80 percent by weight phenyl groups. The composition has the properties of optical clarity as measured by 90% or greater transmission of light of 400 nm wavelength in a 1.0 centimeter path, a refractive index at 589 nm of 1.545 or greater and still has 90% or greater transmission of light of 400 nm wavelength in a 1.0 centimeter path after exposure to 150.degree. C. for 6 hours. The composition is well suited for application as an optically transmissive cover or lens for light emitting diodes and for use in other optical and related devices.