We describe the chemical and electromagnetic enhancements of surface-enhanced resonance Raman scattering (SERRS) for the pyridine molecule absorbed on silver clusters, in which different incident wavelength regions are dominated by different enhancement mechanisms. Through visualization we theoretically investigate the charge transfer (CT) between the molecule and the metal cluster, and the charge redistribution (CR) within the metal on the electronic intracluster collective oscillation excitation (EICOE). The CT between the metal and the molecule in the molecule–metal complex is considered as an evidence for chemical enhancement to SERRS. CR within the metal on EICOE is considered as an evidence for the electromagnetic enhancement by collective plasmons. For the incident wavelength from 300 to 1000 nm, the visualized method of charge difference density can classify the different wavelength regions for chemical and electromagnetic enhancement, which are consistent with the formal fragmented experimental studies. Copyright © 2008 John Wiley & Sons, Ltd.