Cycloaliphatic epoxy oligosiloxane resins with a high degree of condensation (> 85%) were synthesized by a nonhydrolytic sol-gel reaction using 2-(3,4-epoxycyclohexyl) ethyltrimethoxysilane (ECTS), diphenylsilanediol (DPSD), and triphenylsilanol (TPS). Cycloaliphatic epoxy hybrimers with 2 mm thickness fabricated by thermal curing of cycloaliphatic epoxy oligosiloxane resins with a hardener and catalyst were optically transparent (similar to 90%) with a high refractive index of up to 1.583. The fabricated hybrimers also show high thermal resistance having no yellowing during thermal aging at 120 degrees C for 1008 h and a high decomposition temperature (> 300 degrees C). On the strengths of these characteristics, the hybrimers are expected to find application as LED encapsulants. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 122: 2478-2485, 2011