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We have shown that poly(para‐phenylene) (PPP) can be obtained either n‐type or p‐type by ion implantation at low energy (E ≦ 50 keV); PPP is primarily an insulator pellet obtained from compacted powder synthetised by the Kovacic method. To compare with the chemical doping effect, we have studied the conductivity and thermopower of PPP samples after two successive ion implantations with Cs+ and I+.The experimental results show that we clearly obtain reversible doping only in the case of an initially I+‐doped sample: the thermopower sign is changed after a Cs+ implantation with a fluence equal to 3 × 1014 ions cm−2. In the other case (Cs+ initial implantation) we observe the change in thermopower sign at higher fluence (2 × 1016 I+ ions cm−2). This last effect can be attributed to a metal transition induced by the accumulation of defects in the material because of too high implantation parameters (graphitisation).

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