Thin films of various compositions of VzTi1-zOy have been deposited by RF-diode sputtering. A variety of optical and electrochemical measurements have been carried out to determine their suitability to perform as counter electrodes (CE) in electrochromic windows. It has been found that in general, the films containing higher vanadium concentrations have higher charge capacities, which are seen to be easily sufficient to make useful CE layers in electrochromic windows. The electrochromic behaviour of the VzTi1-zOy films themselves is seen to be quite complicated, and does not fit a first order theory. A simple theoretical model based on electrons being localised preferentially on vanadium atoms is given, which goes some way towards explaining this behaviour.