Epoxy siloxane monomer, 1,3-Bis[2-(3-{7-oxabicyclo[4.1.0]heptyl))ethyl}]-tetramethyldisiloxane, was cured with methylhexahydrophthalic anhydride, and the catalysts, N,N-dimethylbenzylamine (BDMA) and tetra-n-butylphosphonium o,o-diethylphosphorodithioate (PX-4ET), were compared. The curing reactivity of BDMA was higher than that of PX-4ET, but the thermal stability of the polymer was lower. PX-4ET caused less thermal discoloration, which increased in proportion to catalyst concentration. The optimum was 0.71-0.35 mol%. Maximum hardness and glass transition temperature as well as minimum coefficient of thermal expansion and thermal discoloration was achieved with equivalent amounts of epoxy and anhydride. (c) 2005 Wiley Periodicals, Inc.