This study investigates the significance of parasitic currents at a WO3 electrode, during cycling in a lithium non-aqueous electrolyte, which do not correspond to reversible ion-insertion reaction but result in a distortion of the expected Faradaic mass/charge relation as well as destruction of material. A measurement of the optical transmission monitors the lithium content of the electrode during cyclic voltammetry. During a cycle beginning and ending at the same transmission Value the parasitic charge is equal to the charge imbalance during the cycle. A mathematical algorithm is then used to determine the parasitic current at each point in the cycle. The results indicate a predominant cathodic parasitic reaction with a Tafel slope of 300 mV per decade, reaching 0.1 mu A cm(-2) at 2 V versus Li+/Li.