With the aid of a home-made high-pressure measuring cell, connected to a commercial power compensated DSC, it is possible to investigate the melting and crystallization behaviour of polymeric materials as functions of pressure (up to 500 MPa). Results from poly-4-methylpentene-1, syndiotactic polystyrene, and different types of polyethylene are presented and discussed. In addition the melting behaviour of alkanes and alkane mixtures has been investigated, and a thermodynamic description is given. These results are helpful for better understanding of phase transition behaviour under pressure. High pressure DSC is a powerful method which gives additional insight into polymer systems.