The present invention provides a process for purifying a monoterpene or sesquiterpene having a purity greater than about 98.5% (w/w). The process comprises the steps of derivatizing the monoterpene (or sesquiterpene) to produce a monoterpene (or sesquiterpene) derivative, separating the monoterpene (or sesquiterpene) derivative, and releasing the monoterpene (or sesquiterpene) from the derivative. Also encompassed by the scope of the present invention is a pharmaceutical composition comprising a monoterpene (or sesquiterpene) having a purity greater than about 98.5% (w/w). The purified monoterpene can be used to treat a disease such as cancer. The present monoterpene (or sesquiterpene) may be administered alone, or may be co-administered with radiation or other therapeutic agents, such as chemotherapeutic agents.