Dielectric studies on the static and complex permittivity have been performed on trans,trans-4'-n-pentyl-bicyclohexyl-4-carbonitrile (5 CCH) in the pressure range 0.1-200 MPa, the frequency range 1 kHz-13MHz and the temperature range 300-390 K. Phase transitions between the isotropic, nematic, smectic B (metastable), and crystalline state have been recorded as steps in the static permittivity. Thus the phase diagram of 5 CCH was established. In particular the slow relaxation process in the nematic and smectic phase has been investigated. The dielectric relaxation time τ || increases by a constant factor of 210 on passing the nematic-smectic B phase transition. From the pressure and temperature dependences of τ we have calculated the activation volumes (Δ ≠ V/cm 3 mol -1 = 88-65 (nematic), = 70-45 (smectic) and ≈ 32 for the isotropic phase) and activation enthalpies (Δ ≠ H/kJ mol -1 = 76-60 (nematic), ≈ 60 (smectic) and ≈ 37 for the isotropic phase), respectively, which are compared with previous results on similar homologous series. The activation parameters increase significantly, when the rigid phenyl ring is replaced by the more flexible cyclohexyl ring.