In this paper we report the functionalization of conductive polypyrrole (PPY) films via electrochemical reduction of the aryl diazonium salts in a manner that is similar to the one employed for other conductive surfaces. To understand the general trends of the grafting behavior of diazonium salts and to establish the optimal conditions for the covalent functionalization of the PPY films, we have compared the grafting behavior of four p-substituted phenyldiazonium salts: p-nitrophenyl diazonium tetrafluoroborate (PNBDBF4-), p-tolyl diazonium tetrafluoroborate (TDBF4-), p-fluorophenyl diazonium tetrafluoroborate (FPDBF4-) and 4-diazo-N,N-dimethylaniline tetrafluoroborate (DDMABF(4)(-)). The selection of the molecules to be grafted was done both for their electroactivity after grafting and the contrasted electronegativity of the substituents at the benzene ring. For all investigated diazonium salts, a linear relationship between their reduction potential at the PPY electrodes and Hammett substituent constants was obtained, suggesting a similar electrochemical reaction mechanism. The functionalization of the polypyrrole films has been evaluated using electrochemical methods like EQCM, CV and EIS. The presence at the polymeric films surface of the functional groups introduced by the electrochemical reduction of diazonium salts was evidenced also by XPS. This approach enables new functionalities on PPY that could otherwise not withstand the polymerization conditions. (C) 2014 The Electrochemical Society. All rights reserved.