The effect of allylthiourea on the spectral sensitization of silver chloride in the electrochemical system has been studied. The following electrochemical cell was constructed with a thin crystal of silver chloride: Pt (I)/electrolyte (I), dye/AgCl/electrolyte (II)/Pt (II). When allylthiourea was added into the electrolyte solution (I) containing 1, 1'-diethyl-2, 2'-quinocyanine chloride as a spectral sensitizer, the sensitized photocurrent by the dye decreased, whereas in the case of the addition of allylthiourea into the electrolyte solution (II), the sensitized photocurrent increased. These phenomena were interpreted in terms of the formation of silver sulfide, which was expected to introduce a deep electron trap, on the surface of the silver chloride crystal.