A theoretical model is presented to describe the chemical mechanism of surface-enhanced resonance Raman scattering (SERRS) via charge transfer (CT) in the pyridine–Ag2 complex. We first describe the influence of the interaction between the metal cluster and pyridine to the ground-state properties of the pyridine–Ag2 complex, such as charge redistribution, the change of the atomic-resolved density of state, and the change of energy levels of occupied and unoccupied molecular orbitals. Second, we visualize the CT between the metal cluster and pyridine and within the intracluster on the electronic state transitions with charge difference density. The CT between the metal cluster and pyridine is the direct evidence of chemical mechanism for SERRS. Third, the spectra of SERRS are calculated with different incident light wavelengths that resonate with the different electronic state energy levels, and the enhanced intensities of different vibrational modes are compared, which show that there are different enhancement rates for different vibrational modes. Strong Raman scattering can be achieved not only by the CT between pyridine and the metal cluster but also by electronic intracluster excitation via a type of Förster excitation transfer, and the latter results from the local field effects by collective plasmons. The selection rules for the SERRS have been obtained for these two types of enhanced mechanisms. Copyright © 2008 John Wiley & Sons, Ltd.