The reversibility of the doping reaction with iodine of Shirakawa polyacetylene (PA) films, carried out in the gas phase, was studied by means of elec. cond., IR, and ESR measurements. As expected, the level of the doping strongly influences the rate of the back reaction, diffusion processes being involved through the various parts of the film. These processes are characterized by diffusion consts. which differ by orders of magnitude. For lightly doped PA samples, it is easily possible to recover the initial characteristics of the elec. cond., while the optical IR characteristics may be recovered by heating the samples at 50°, if both cis and trans isomers are present. For lightly doped all-trans samples, this heat treatment was not necessary. The heavily doped all-trans PA samples also regain their initial characteristics after brief heating, showing that the desorption of I2 from the interior of the film needs to be thermally activated.