The coupled-oscillator model, previously used to describe linear optical activity, is extended to the study of nonlinear optical activity of dimer-like chiral molecules. The result shows good agreement between theory and experiment for optically active sum-frequency generation from 1,1'-bi-2-naphthol solution. It is seen that linear and nonlinear optical activities generally probe different aspects of the chiral structure of the molecules. (C) 2002 Elsevier Science B.V. All rights reserved.