The influence of a halogen-gas (Cl-2 or F-2) treatment on both structural and physical properties of La2CuO4 has been studied. In ail cases, the resulting product contains an excess of anionic species and shows superconducting properties below 32K and 40K respectively for the chlorine- or fluorine-treated sample. But these treatments have a different influence on the evolution of the : structural properties comparatively to those observed for the starting oxide : after chlorination a decrease of the orthorhombic distortion is detected, whereas an opposite trend is shown after fluorination. The Cl-2-gas treatment leads to an oxygen-excess phase La2CuO4.08, whereas the fluorination yields an oxyfluoride La2Cu(O,F)(4.17).