We demonstrate a novel method to tune the energy gap 1 between the localized states and the mobility edge of the valence band in chemically functionalized graphene by changing the coverage of fluorine adatoms via electron-beam irradiation. From the temperature dependence of the electrical transport properties we show that 1 in partially fluorinated graphene CF0.28 decreases upon electron irradiation up to a dose of 0.08 Ccm−2. For low irradiation doses (<0.1 Ccm−2) partially fluorinated graphene behaves as a lightly doped semiconductor with impurity bands close to the conduction and valence band edges, whereas for high irradiation doses (>0.2 Ccm−2) the electrical conduction takes place via Mott variable range hopping