Ag HgClI 2 and Ag HgCl 2I were obtained by the interaction of HgCl 2 and AgI in the solid state at 100°C. A diffusion-controlled HgCl 2AgI reaction proceeds via counterdiffusion of Ag + and Hg 2+ and its activation energy is 23.90 kcal/mole. At the start, the process is surface or grain boundary diffusion-controlled having an activation energy of 8.32 kcal/mole. Different molar ratios of the reactants gave rise to different end products. The reaction was followed by X-ray, chemical analysis, thermal, conductivity measurements. Evidence for the formation of solid solution between Ag 2HgI 4 and AgCl is reported.