Ring-current maps for CAl42- and CGa42- are calculated, and the origin of the ring current in these systems is discussed and compared with those in Al-4(2-) and Ga-4(2-). Differences between the systems can be rationalised by examining the changes in molecular orbitals, caused by adding the central carbon atom. The current is mainly dominated by the response of the four electrons in the B-2g and B-1g orbitals and as with those of the all metal systems, the currents from the pi-electrons are negligible, classifying these clusters as sigma aromatic, like their `empty' analogues. (C) 2013 Elsevier B.V. All rights reserved.