The oxidation of BF4--doped poly[(1,4-dimethoxybenzene)-co-(3-methylthiophene)] (PDMB-PMeTh) at 1.2 +/- 0.1 V (vs. 10(-1) M Ag+Ag) in dry CH3CN + 0.2 M NEt4CN leads to a new CN-substituted copolymer. Comparative to the PDMB homopolymer, this electrochemical reaction is more efficiently performed, owing to better film properties provided by PMeTh. The electrochemical and XPS data show that the anodic cyanation is achieved specifically on PDMB and results mainly in the aromatic substitution of hydrogen on the DMB unit rather than the replacement of aromatic methoxyl by nitrile. Thus the cyanation mechanism would be different from that demonstrated in solution with DMB, for which p-methoxybenzonitrile was generated. The ratio of the MeTh and DMB units in the copolymer is estimated at 0.4 +/- 0.05 and the cyanation results in the linkage of 1.4 +/- : 0.2 C = N group per DMB unit. Moreover, this study reveals that cations and anions from the electrolyte are strongly retained in the copolymer and partially exchanged after the electrosynthesis, cyanation and chemical treatment steps.