Results of ESR measurements on iodine-doped polyacetelene are reported. With increasing iodine content the number of paramagnetic centres decreases to a few per cent of the original number and the linewidth increases strongly. When the iodine is removed by heating up to 200 °C, the state before doping is recovered to a certain extent. The number of paramagnetic centres reaches nearly the value before doping, whereas the linewidth does not, indicating irreversible changes in the samples during the doping process. The results are consistent with the soliton doping mechanism and with the concept of highly mobile and fixed spins coexisting in polyacetylene.