Creating high refractive index (RI) thermally stable polymers for encapsulating high-brightness light-emitting diodes (LEDs) remains a challenge and is an opportunity for improving LED efficiencies. The best previously reported RI for a 200°C heat stable encapsulant for LEDs is 1.56. Here, we report the use of novel phenoxyphenyl and phenylthiophenyl silicone monomers to give fully formulated encapsulants with RIs above 1.60. These liquid dispensed encapsulants are highly heat stable, showing little change in optical properties after heat aging at 200°C in air for seven weeks, and were also little changed after cycling between −10°C to 85°C over 6 months. Phenoxyphenyl(phenyl) dimethoxysilane and phenylthiophenyl(phenyl) dimethoxysilane monomers were prepared via Grignard reactions. The resulting monomers were copolymerized with commercial silicone monomers and incorporated into hydrosilation-based thermosets designed for use as LED encapsulants. RIs for the cured polymers were 1.60 at 633 nm (1.62 at 450 nm) for the phenoxyphenyl ether system and 1.62 at 633 nm (1.65 at 450 nm) for the phenylthiophenyl ether system. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 39824.