Electrochromic devices of the general type indium tin oxide (ITO)WO3polyethylene-oxide (PEO)-H3PO4(H)ITO were constructed. The mechanism of electrocoloration involves protons decanting from the pre-intercalated ITO to form the cell ITOHxWO3PEO-H3PO4ITO. During potentiostatic coloration, the current goes through a minimum and a maximum at short times. Simultaneous monitoring of cell transmittance shows that the processes causing the current peak are colour forming in origin. It is suggested that the peak is caused by an electronic mobility threshold.