Thin polymeric films of diphenyl were prepared by a plasma polymerization technique using a capacitively-coupled reactor. The dielectric relaxation spectroscopy was employed to investigate the behaviour of the as-deposited and heat-treated plasma-polymerized diphenyl (PPDP) thin films in a aluminium/PPDP/aluminium configuration over the frequency range from 10(-1) to 10(6) Hz and temperature range from 223 to 423 K. The ac conductivity is more dependent on temperature in the low frequency region than in the high Frequency region. In these materials the conduction may be dominated by hopping of carriers between the localized states at low temperatures and thermally excited carriers from energy levels within the band gap in the vicinity of high temperature. Dielectric constant is dependent on frequency above 303 and 343 K in the as-deposited and heat-treated PPDP, respectively. The as-deposited PPDP shows superposed alpha and beta relaxation processes. For different relaxation processes, the activation energies are estimated to be about 0.45 and 0.67 eV for the as-deposited and 1.35 eV for heat-treated PPDP. The dielectric data analyses show the existence of distribution of relaxation time in these materials.