Electrochemical quartz crystal microbalance (EQCM) measurements were used to study redox-induced changes in the ion and solvent content of polybithiophene/polyxylylviologen (PBT/PXV) bilayers. Experimental measurements were taken under slow scan voltammetric conditions in 1 mol dm(-3) tetraethylammonium perchlorate + CH3CN. Outer (PXV) layer redox conversion occurs only at potentials where the inner (PBT) layer becomes conducting. On the relatively long time-scales employed, bilayer mass changes are dominated by perchlorate anion transfer (ejection on PXV(2+) reduction and entry on PXV(0) oxidation), with relatively little solvent transfer. The mass-charge characteristics of PBT/PXV bilayer and PXV single-layer films are relatively similar, despite their very different mass-potential characteristics.