Abstract: The present invention provides an encapsulant for optical semiconductor devices, which is capable of suppressing surface tackiness of a cured product and is also capable of enhancing the heat resistance and thermal cycle characteristics of the cured product. An encapsulant for optical semiconductor devices according to the present invention which includes: a first organopolysiloxane which is represented by formula (1A) or formula (1B) and has an alkenyl group and a methyl group bonded to a silicon atom; a second organopolysiloxane which is represented by formula (51A) or formula (51B) and has a hydrogen atom bonded to a silicon atom and a methyl group bonded to a silicon atom; and a catalyst for hydrosilylation reaction, and the content ratios of methyl groups bonded to silicon atoms in the first and second organopolysiloxanes each are 80 mol % or more.

Abstract: The present invention provides a sealant for an optical semiconductor device which has high gas barrier property against corrosive gas, and is less likely to crack or is less likely to peel off even when used in harsh environments.