To clarify the mechanism of thermal dedoping processes of conducting poly(3-alkylthiophenes), temporal changes in electric conductivity, photoabsorption, and electron spin resonance of iodine-doped poly(3-octylthiophene) were examined. A decrease in the electric conductivity was accompanied by a decrease in the photoabsorption of the polymer cation and an increase in the spin concentration: the cation is a spinless charge carrier and it was converted to an immobile paramagnetic species. The conversion proceeds through deprotonation of the cation, which results in the formation of a polyenyl radical.