First principles calculations have been used to investigate the effect of fluorine substitution on the electrochemical properties of Li2VOSiO4 and LiFePO4. Several models of hypothetical LiVSiO4F and Li0.5FePO3.5F0.5 within the structure of the corresponding parent compounds have been analyzed. The computational results indicate that the lithium insertion voltage is largely controlled by the crystallographic site that fluorine ions occupy in the structure. In some models of LiVSiO4F and Li0.5FePO3.5F0.5 lithium deinsertion causes a too large M–F distance (indicative of MF bond breaking), being the predicted lithium insertion voltage about 0.3V lower than that of the parent compound. In the energetically most stable model of hypotetical-LiVSiO4F the VF bond is retained, and a voltage increasing of 0.5V is predicted with respect to that of the parent compound. For hypothetical Li0.5FePO3.5F0.5 models consisting on FeO6 and FeO4F2 octahedra the lithium deinsertion produces solely the oxidation of Fe2+ surrounded by oxygen ions, which is accompanied by a voltage decreasing. These findings are supported by the computational results on the known intercalation compound LiVPO4F.