We have calculated the first-, second-, and third-order contribution to the refractive index of a dilute aqueous solution of three-level molecules irradiated by an intense one-frequency external field. Each solute molecule is described by two coupled harmonic curves of electronic energies with the same force constant and with minima displaced in energy and nuclear coordinate. We assume that the intramolecular coupling mixes only the states corresponding to two near degenerate of the three levels considered. Important linear and nonlinear contributions of the solute molecules to the refractive index have been found, mostly in the so-called weak and intermediate coupling regions. © 1993 John Wiley & Sons, Inc.