Li+ charge transfer and the colouration dynamics in an electrochromic tungsten trioxide film electrode have been studied by the potentiostatic intermittent titration technique (PITT) and simultaneous in-situ optical transmittance at one fixed wavelength (670 nm). Relaxation transients of the optical density and of the total integrated charge after potential steps have been measured in intercalation as well as in deintercalation up to a Li composition x of 0.4. The experimental colouration efficiency (CE) (≤80 cm2 C−1) has been deduced as a function of x in such experiments and compared with the efficiency calculated according to the site-saturation model. The small departure of the data from the calculated efficiency was explained in terms of a partially irreversible cathodic reaction, which accounts also for some charge retention in the electrode after a complete cycle. Both charge intercalation and colouration dynamics appeared to follow a diffusion controlled process with a diffusivity of the order of 5×10−11 cm2 s−1. The optical data collected in a complete charge–discharge cycle showed a smaller hysteresis than the electrochemical data, and the colour diffusivity was always higher than the charge diffusivity, in particular at the end of either insertion or deinsertion processes (large x and small x, respectively). Such a small but systematic discrepancy between the optical and electrochemical kinetic data suggests that part of the charge is diverted from the main electrochromic process into a secondary electrochemical process.