The irreversibility of the doping-undoping processes and its effect on the electrochemical and photoelectrochemical behaviour of conducting polymers was studied for a series of polythiophene derivatives: polybithiophene, poly-3-methylthiophene and poly-3-phenylthiophene. For the three polymers it was confirmed that both anodic and cathodic undoping processes involve a considerable amount of residual charge trapped in the polymer phase. The nature of charge trapping for all the polymers was related to the incompleteness of the undoping processes rather than the chemical modification of the polymer, although the latter was also shown to occur. It was demonstrated that the trapped charge can be removed not only electrochemically (at the beginning of the polymer doping process of the opposite sign), as has been shown earlier, but also photoelectrochemically, by illuminating the polymer in solution at appropriate electrode potentials where the cathodic photocurrents are observed. The latter fact suggests that the photoelectrochemical undoping of these polymers observed by us earlier is in fact related to the photoassisted removal of the trapped charge. (C) 2000 Elsevier Science B.V. All rights reserved.