One of the key issues associated with epoxy resin-based LED encapsulant is discoloration under operating conditions that caused a decrease in light output. In this article, thermal cationic polymerization of hydrogenated bisphenol-A glycidyl ether using 3-methyl-2-butenyltetramethylenesulfonium hexafluoroantimonate and its discoloration are reported. Polymerization was followed by thermal analysis, FT-IR spectrum, and hardness measurements. Thermal discoloration was measured by yellowness index based on transmittance spectrum. Increased catalyst concentration causes acceleration in polymerization rates, but this also increased discoloration after thermal aging. In low catalyst concentration under 0.3 wt %, the hardness of plaque was low and required a high temperature to complete polymerization and this resulted in a highly colored plaque. Formulation with cycloaliphatic epoxy (3'-4'-epoxycyclohexane)methyl 3'-4'-epoxycyclohexyl-carboxylate (CEL2021P) enhanced the polymerization even at low catalyst concentration and reduced thermal discoloration. With addition of di-t-butylphenol (BHT) as the oxidation inhibitor, the thermal discoloration was extremely improved but this also caused inhibition of polymerization as seen by thermal analysis. (c) 2005 Wiley Periodicals, Inc.