Communication: One-step post-polymerization-functionalization of polythiophene is reported. Using the new method presented here a large variety of amines carrying, for example, sterically demanding or electronically sensitive functional groups can be attached to conducting-polymer-modified electrodes even under aqueous conditions. The Figure shows the starting material which is polymerized then functionalized. The following paper in this issue presents the use of these materials in the development of amperometric biosensors.