Electrogenerated deposits of poly[4,4'-bis(butylsulphanyl)-2,2'-bithiophene] have been characterised in the presence of different supporting electrolytes, by coupling current (charge) to microgravimetric measurements from the electrochemical quartz crystal microbalance (EQCM). The simultaneous collection of voltammetric measurements and of data relative to mass changes shows the influence exerted by the nature of the supporting electrolyte on the charge-discharge steps of both p- and n-doping processes. Interestingly, the microgravimetric data collected corresponding to the pre-peaks of both doping processes suggest the occurrence of two opposite motions of ions (ingress into and exit out from the polymer) in the relevant potential region, giving a contribution to the study of the 'residual charge' phenomenon. (C) 2004 Elsevier B.V. All rights reserved.