The effect of magnetic field on the luminescence of pyrene-DMA exciplex confined within the reverse micellar (RM) environment of various cavity sizes is reported. The field-modulated change in luminescence Deltaphi/phi) at various w values is strikingly similar to the Deltaphi/phi vs dielectric constant (epsilon) variation of solvent mixtures which suggests that epsilon of the RM interface is primarily responsible for the variation of magnetic field effect (MFE). The interfacial dielectric constant at different w values may be evaluated on the basis of the above similarity. (C) 2004 Elsevier B.V. All rights reserved.