Absorption properties of aggregated molecular systems, which are not necessarily chemically bonded, can be analysed in terms of exciton phenomena. Angular momentum terms arising from exciton coupling often dominate the natural circular dichroism (CD) of molecular aggregates. This Letter, for the first time, extends such an analysis to the magnetic circular dichroism (MCD) of exciton-coupled molecular dimers. In particular, we determine the contribution to the Faraday B term MCD of an arbitrary dimer. A reduction in the MCD/absorbance ratio has been reported for various aggregated chlorophyll-like systems. Our analysis suggests that such observed variations in the B terms are not easily attributed to exciton coupling. (C) 2003 Elsevier B.V. All rights reserved.