The solid-phase halogenation for a conducting polymer, poly(thiophene-alt-dioctylfluorene) (PTF), was carried out by constant current electrolysis in an acetonitrile solution containing tetraethylammonium halide using the polymer film on the anode. A cyclic voltammetric analysis for PTF and potentiostatic studies disclosed the reaction mechanism for the electrochemical polymer reaction. An electrolysis of the PTF film on a transparent electrode such as indium tin oxide (ITO) was also carried out.