A high pressure cell for a power-compensated DSC is presented, allowing measurements at pressures up to 500 MPa. With the aid of this apparatus, n-alkanes, binary n-alkane mixtures, and two linear polyethylenes have been investigated. The fusion temperature and the heat of fusion have been measured for different n-alkanes in the pressure region between normal pressure and up to 500 MPa. For five eutectic and non-eutectic binary mixtures, the liquidus and solidus temperatures were determined. It is possible to describe the mixing behaviour of n-alkanes with the one-parameter approximation derived by Porter. The increase of the Porter parameter with pressure implies decreasing miscibility at higher pressures. From the Porter parameter the critical point can be calculated. Thus, it can be seen that n-alkanes which build mixed crystals at normal pressure demix at higher pressures. From these results, it can be concluded that the same is true for polyethylene.