Pressure dependence of d.c. conductivity of poly(3-octylthiophene) (POT), poly(3-decylthiophene) (PDT) and poly(3-dodecylthiophene) (PDDT) has been measured under hydrostatic pressure up to 6 kbar as a function of FeCl3 doping. In all cases conductivity increased with pressure. The increase depends on the ambient pressure conductivity and side chain length. A maximum in the pressure dependence was found in the low conductivity region. With increasing side chain length, the pressure dependence increased and its maximum shifted to higher conductivity. At dilute doping levels Kivelson's theory for intersoliton hopping was applied. The three-dimensional variable range hopping model was used in the intermediate doping region. The Kivelson theory was applied to give numerical values for the bulk compressibility ranging from 2 × 10−10 m2/N (POT) to 6 × 10−10 m2/N (PDDT). In the metallic region the measured pressure effect on conductivity is small and less dependent on the side chain length.