Coumarin 500 exhibits dual emission in the reverse micellar system of heptane/AOT/water, whereas the structurally similar analog Coumarin 151 exhibits single emission. Steady state and time resolved data suggest that the difference in emission behavior results from unequal partitioning of the dyes between the heptane and aqueous phase. Kamlet-Taft solvatochromic analysis of the absorption maxima of the dyes in different solvents indicates that for C151, hydrogen bonding with the protic solvent water imparts additional stability, which accounts for its greater partitioning into the aqueous phase. (c) 2005 Elsevier B.V. All rights reserved.