A thermally cured epoxy-siloxane hybrid material that is curable at low temperature (L-expoxy hybrimer) was investigated for use as an LED encapsulant. This new hybrimer was fabricated using thermally initiated, cationic polymerization of cycloaliphatic epoxy oligosiloxane (CAEO) resin, derived from non-hydrolytic sol–gel, mixed with oxetane hardener in the presence of a hexafluoroantimonate-type thermo-cationic initiator. The L-epoxy hybrimer was cured at a lower temperature (below 120°C) than previously reported for an epoxy hybrimer with anhydride hardener (above 180°C). The L-epoxy hybrimer showed high thermal resistance to yellowing under long-term high temperature condition, and maintained good optical transmittance. Also, it had a high refractive index (up to 1.57), as well as the hardness (Shore D 80), and low water-vapor permeability, when the new hybrimer was used to encapsulate an LED, it showed good adhesion without cracks or delamination and maintained their initial performance after the long-term aging tests (120 and 85°C at 85% humidity). © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2014, 131, 39968.