Transition-metal-doped semiconductor nanoparticles (NPs) have been well studied for their optical and catalytic properties but seldom studied by surface-enhanced Raman scattering (SERS). In this paper, transition-metal-doped semiconductor NPs are investigated for their SERS property. Four groups of Co-doped (0.5, 1, 3, and 5%) ZnO (CoZnO) NPs and pure ZnO NPs were synthesized and studied. When 4-mercaptobenzoic acid was used as probing molecule, significant SERS signals were obtained on all the five samples. Moreover, it is very interesting to observe a relationship between the Co-doping concentration and enhancement of the SERS signals. SERS intensities first increase with doping concentration (up to 1%), and then decrease with further increase in doping concentration (up to 5%). Charge transfer (CT) is considered to be the main contribution to this phenomenon. Different CT ratios from substrates to molecules seem to induce different intensities of the SERS signals. In our experiments, the crystalline defects of CoZnO NPs caused by the Co dopant affect the CT ratios. A possible mechanism of CT from the valance band of CoZnO NPs to the lower unoccupied molecular orbital of the molecules via energy of the surface states is suggested. X-ray photoelectron spectra, UVvis spectra, and Raman spectra were used to characterize the structure and defects in CoZnO NPs. Copyright © 2011 John Wiley & Sons, Ltd.