The surface enhanced Raman spectra of ethylene thiourea adsorbed on the silver electrode were measured at different electrode potentials. Based on the analysis of the Raman spectra, especially the bond polarizability derivatives elucidated from the Raman intensities by our algorithm, important information on the surface enhancement mechanisms was collected. The results show that both the electromagnetic and charge transfer mechanisms are responsible for the large enhancement, the former operating in the whole potential range between -0.3 and -1.2 V while the latter only in the range lower than -0.7 V.