We propose a model for a sulfur-plus-gold sensitization center, which is derived by replacing one of two interstitial silver ions with a monovalent gold ion in a model for sulfur sensitization center proposed in this series of papers, and predict that (a) the incorporation of gold ions into silver halide grains is enhanced by the presence of sulfur sensitization centers owing to stronger bond formation of sulfide ions with gold ions relative to that with silver ions, and that (b) an electron trap for a sulfur-plus-gold sensitization center is shallower and has larger cross section than that fur a sulfur sensitization center, since a gold ion is larger than a silver ion. The prediction (a) was supported by the measurement of the amount of gold ions incorporated into AgBr grains by means of an isotopic tracer technique with a gold sensitizer labeled with ¹⁹⁸Au. The prediction (b) was supported by the measurements of the activation energy and frequency factor of the rate of development of developer fog, which resulted from the electron transfer from a developer to sensitization centers.