SERS of trans-3-hydroxycinnamic acid adsorbed on silver colloids have been analyzed in order to detect resonant charge transfer processes similar to a resonance Raman mechanism, involving the photoinduced transfer of an electron from the metal to the adsorbate (SERS-CT). Theoretical SERS-CT intensities for the neutral, anionic and dianionic forms have been calculated and it was found that the active SERS species corresponds to cinnamate ion linked to the metal through its carboxylate group. Moreover, it has been shown that the SERS-CT mechanism is operating in the SERS, explaining the strong enhancement of the band at ca. 1600 cm−1, assigned to the 8a mode.