With the integrating sphere device, in situ UV–Vis diffuse reflectance spectra were obtained for the deintercalation process of a lithiated petroleum coke electrode. In the lowest frequency region studied the reflectance was found to increase with increasing lithium content, whereas little change was seen in the π–π* transition region. According to a simplified simulation, the spectral changes are thought to be associated mainly with the change of the number of free charge carrier electrons.