To obtain a new peroxide useful as a curing agent for a thermosetting resin, capable of curing a molding material in an increased cure rate and of reducing color development in cure and molding, and of reducing yellowing and a change of color of a molded product in service.

This compound is a peroxide expressed by the formula (R1 is a 5-8C tertiary alkyl), e.g. tertiary amyl peroxy 3,5,5-trimethylhexanoate. A compound of the formula is obtained by condensation reaction of a hydrogen peroxide such as 2,4,4-trimethylpentyl hydrogen peroxide, t-hexyl hydrogen peroxide, t-amyl hydrogen peroxide, etc., with 3,5,5-trimethylhexanoly chloride in the presence of an alkaline catalyst such as NaOH at ≤20°C for 1-2hr.