The stereochemistry of the cometathesis of cyclododecene (CDD) with hexene-1 in the presence of the MoCl5/SiO2-Me4Sn heterogeneous catalytic system has been studied. It has been established that CDD is the mixture of cis- (Z) and trans- (E) stereoisomers with the ratio of 30/70, respectively. It has been shown that the reactivity of the E-stereoisomer of CDD in the reaction of cometathesis is higher when compared to the Z-stereoisomer. Thus, when the conversion of CDD is 92%, the ratio E/Z = 6 : 94, i.e., the amount of the E-stereoisomer decreases from 70 to 7%, while the amount of the Z-stereoisomer increases from 30 to 93%, respectively.