Thus, the liquid phase was assured throughout the experiment. However, the working conditions are very close to the critical region of pure butenes, the critical points of butenes being (Tsonopoulos and Ambrose, 1996): 419.6 K and 4.02 MPa for 1-butene; 428.6 K and 3.99 MPa for trans-2-butene; and 435.6 K and 4.20 MPa for cis-2-butene.