An apparatus based on Bridgman's bellows method, originally devised for fluids, has been constructed for use with solid samples. Mercury is employed as the confining liquid. The operating range is 0≤t≤200 °C and 1≤P≤2000 bar. Calibration with benzene and mercury shows an accuracy of ±2×10−4cm3/g in the measurement of the specific volume change. A detailed investigation of two systems, polystyrene and poly(orthomethylstyrene) in the glassy and liquid states is undertaken. TheP‐V‐Tresults in both states can be well represented by the Tait equation, with the single parameterBas an exponentially decreasing function ofT. Similar to the earlier result for polystyrene, a &bgr;‐relaxation region is observed in terms of the isothermal compressibility of the methylated polymer withT&bgr;/Tg≃0.70 for both. The pressure coefficients for the two transition temperatures are discussed. For the glass transition, the Ehrenfest‐type equation appears to be obeyed by the low‐pressure glasses (formed by cooling the liquid at atmospheric pressure) but not the high‐pressure glasses (formed by pressurizing the melt isothermally). The experimental compressibility factors, the internal pressures, and the changes in internal energy upon compression are compared with the predictions of the cell theory of the Brussels group and the hole theory of Simha‐Somaynsky. The characteristic parameters are evaluated by a superposition of the experimental and theoretical atmospheric‐pressure isobars and from the ordinates of one isotherm. In the temperature and pressure range considered here, the performance of the cell theory is quite satisfactory. Very good agreement in respect to theP‐V‐Tfunction is obtained for the hole theory.