A study of the effects of irradiation by energetic carbon ions (56 MeV C6+) on the transport properties of polyacetylene is presented. The in situ monitoring of the resistivity ρ shows an initial nonlinear increase by a factor of 104 as a function of the integrated flux of particles Φ. For trans-(CH)x, the increase in ρ saturates at a plateau starting at Φ0≃5×1012 particles/cm 2, whereas trans-(CD)x behaves differently. After a similar (but slower) initial increase, ρ of trans-(CD)x peaks at Φ0 and subsequently decreases for Ω>Ω0. Parallel thermoelectric power data show an increase from ∼880 μV/K to ∼1100 μV/K for Φ≃Φ0 for both isotopes. Characterization studies of the irradiated film using infrared and Raman spectroscopy show no detectable change in backbone structure, and ESR measurements show no increase in the number of unpaired spins. Moreover, after irradiation the polymer can be doped to the highly conducting metallic regime. We therefore conclude that the conjugted (CH)x chains remain intact during irradiation; either the deposited energy is rapidly dissipated to avoid bond breaking or there is a high tendency to self-heal. The increase in ρ is primarily the result of a decrease in the number of carriers. The dramatic differences in the behavior of trans-(CH)x and trans-(CD)x suggest that hydrogen or deuterium is released during irradiation and that these radicals subsequently compensate the residual carriers in the polymer. Aside from this compensation effect, it is concluded that polyacetylene is relatively immune to damage by irradiation. © 1983 American Institute of Physics.