PURPOSE: To obtain a compsn. containing a diadduct in a high ratio by heating a mixture of terpene such as γ-terpinene and Diels-Alder dienophille such as maleic anhydride to form a diadduct.

CONSTITUTION: 2 mol equivalent of Diels-Alder dienophille (pref., maleic anhydride) is used per 1 mol of terpene (pref. containing 95% or more of γ-terpinene) and one of them is heated to reaction temp. and the other one of them is gradually added to react both of them at 140-200°C (pref., 150-170°C) to obtain a reaction product consisting of at least 80% of a diadduct and 20% or less of a monoadduct. The obtained crude reaction product is a light yellow solid and does not show strong coloration and is high in diadduct content and, therefore, purification is relatively easy but considered to be probably unnecessary.