Localised-orbital analysis of the current density induced by a perpendicular magnetic field in square-planar Al-4(2-) demonstrates the intrinsic non-localisability of the sigma electrons of this metallic cluster and confirms their dominant role in its diamagnetic ring current. Though delocalised, the pi electrons do not give rise to a significant ring current. (C) 2002 Elsevier Science B.V. All rights reserved.