A small-angle X-ray scattering study on two different polyethylene samples under pressure up to about 750 MPa reveals a decrease of the Pored invariant Q(p) with increasing pressure at constant temperatures. The volume-pressure relationship as well as the isothermal compressibility and the Gruneisen parameter for a large pressure range and the overall density range are calculated from Q(p) with the help of the Tait equation; From the Porod invariant, the parameter B(t) of the Tait equation for the sample as well as of the purely amorphous and purely crystalline polyethylene are obtained. A general relationship between B(t) and the sample density has been found.