Stimulated fluid flow in geothermal reservoirs can produce surface Self-Potential (SP) anomalies of several mV. A commonly accepted interpretation involves the activation of electrokinetic processes. However we can not rule out electrochemical or electrothermal processes generated by the differences in chemical composition and temperature between the in-situ and injected fluids. We analyzed the relative contribution of electrokinetic, electrochemical and electrothermal potentials to the SP anomalies observed above the Soultz-sous-Forêts geothermal reservoir while injecting cold fresh water. We found that electrokinetic effects were dominant during the stimulation phase (injection rates larger than 30 kg s−1) and that electrochemical and electrothermal effects became significant at small injection rates (less than 10 kg s−1) and after shut-in.