In the years 1978 through 1981, many spontaneous polarization (SP) surveys of porphyry sulfide deposits and prospects were done in the western United States. Effective depth of exploration using SP was found to exceed 1 km for large porphyry sulfide orebodies. The relationship between depth of erosion since time of emplacement and the spontaneous polarization observed on the surface, together with the effects of weathering, are presented from field surveys over known deposits.Successful use of the spontaneous polarization technique in porphyry sulfide exploration led to an investigation of the causative mechanism. The field data do not support the electrochemical model first proposed in Sato and Mooney (1960). The measured potential appears to be simply the oxidation potential Delta E between a reference electrode outside the mineralization and a roving electrode within the mineralization. The Delta E measured in the field is dominated by the difference in Eh (Delta Eh) potential between the two electrodes, but it is not equivalent due to the unavoidable inclusion of other potentials in the field measurement. Without a wire connecting the electrodes, current between the sulfide mineralization and the surrounding country rock is sensibly zero, as evidenced by the persistence of the sulfides through geologic time.