The iodometric titration method was used to study the kinetics of some individual terpenoids epoxidation reactions (α-pinene, limonene, limonene oxide), and the medical-grade turpentine epoxidation with the peroxy decanoic acid in the methylene chloride medium at 297-298 K. The kinetics of the reaction was shown to be governed by the second order kinetic equation. The second order effective constants of the reaction rate were determined: for α- pinene – 3.6 L mol-1 min-1 (297 K), limonene (to epoxide) – 2,1 L mol-1 min-1 (295 K), limonene oxide (to dioxide) – 0.052 L mol-1 min-1 (295 K), for turpentine – 3.9 L mol-1 min-1 (297 K). The proposed techniques of the quantitative determination of the basic substance content and determination of the iodine number of some terpenoids (α-pinene, limonene, limonene oxide) and the (medical-grade) gum turpentine were based on the data of epoxidation with the peroxy decanoic acid in the methylene chloride medium. The extraction-titrimetric technique was practiced and the possibility of quantitative determination of the (medical-grade) gum turpentine in turpentine ointment 10% (RSD = 3.2%, δ = -1.15%) was demonstrated.