A silver halide emulsion which exhibits high sensitivity, high contrast, little sensitivity variation with humidity conditions at the time of exposure, and excellent reciprocity law properties at high illumination intensities. Also, a method of preparing the emulsion in a stable manner, and a silver halide color photographic photosensitive material and an image forming method that use the emulsion. The emulsion includes a mesoionic compound having a thiolate structure or a protonated thiolate structure, and the emulsion is sensitized by an Au (III) compound. The emulsion preferably contains an oxidatively dimerized form of a mesoionic compound having a thiolate structure, and a silver chloride content of at least 90 mol %.