Ion implantation of selected polymer films leads to the formation of semiconductive (σ ~ 10−4 S/cm) derivatives, which are presumably partially carbonacėous products derived from gross structural rearrangements. In support of this contention we find that Br-implanted polyacrylonitrile, PAN, exhibits a narrow ESR signal with a g-value of 2.0033, consistent with the presence of free radicals delocalized within a П-electron system. Thermopower measurements reveal that the sign of the majority carrier is dependent upon the molecular structure of the parent polymer.For example, implantation of PAN with Br+ affords n-type derivatives while under similar conditions poly(p-phenylene sulfide), PPS, and poly(2,6-dimethylphenylene-oxide), PPO, yield p-type semiconductors. It is suggested that the majority carriers arethe ions (carbenium ions or carbanions) which are best stabilized by the parent polymerstructure, remnants of which presumably exist after implantation. Resonance and/or inductive effects are invoked to explain the data. For example, the rather electron-rich backbone of PPQ is expected to stabilize holes more effectively than electrons, and the observed p-type behavior is consistent with this prediction.