The write-erase efficiency of electrochromic cells incorporating heptyl viologen (HV) is decreased if di-reduced HV0 is formed : this is observed as a failure to electro-oxidise the colour of the display. This failure is caused by comproportionation of electrogenerated HV0 with HV2+ (from the solution bulk); comproportionation results in the generation of electrochemically inert radical-cation dimer HV22+. Addition of ferrocyanide to the solution as an electron mediator improves the write-erase efficiency of an HV-based device by promoting a different comproportionation mechanism that does not involve the radical-cation dimer.