The process of the epoxidation of cis,trans,trans-1,5,9-cyclododecatriene (CDT) to 1,2-epoxy-5,9-cyclododecadiene (ECDD) with the 30% aqueous hydrogen peroxide under the phase transfer conditions and with tert-butyl hydroperoxide under the homogeneous conditions was investigated. Onium salts such as Aliquat ® 336, Arquad ® 2HT, methyltrioctylammonium bromide and the Na 2 WO 4 /H 3 PO 4 catalyst system are very active under the phase transfer catalysis (PTC) conditions for the selective epoxidation of cis,trans,trans-1,5,9-cyclododecatiene (PTC method). These catalytic systems were found to be as active and selective as the homogeneous phase system Mo(CO) 6 /TBHP (hydroperoxide method).