Experiments on the Sb-S-Se system were conducted at 300°C, and a continuous stibnite-antimonselite binary solid solution was established. By substituting S for Se, the compositions of S-rich and Se-rich endmembers were confirmed as Sb₂S₃ and Sb₂Se₃, respectively. Based on Se/(S+Se) ratios of microprobe analyses, binary stibnite-antimonselite solid solutions are defined as stibnite, selenium stibnite, sulfur antimonselite, and antimonselite. Microhardness of the stibnite subseries (Sb = 60.11-72.58, S = 13.20-27.63, and Se = 0.00-27.23 wt%) and the antimonselite subseries (Sb = 49.29-59.25, Se = 28.89-51.94, S = 0.00-12.10 wt%) varies from 112.95 to 127.72 kg/mm². The variation of Se concentration is continuous throughout the series, confirming a random substitution of Se for S. Crystallographic parameters obtained from the series vary as follows: a = 1.123375-1.163890 nm, b = 1.132502-1.179553 nm, c = 0.383914-0.398071 nm, D = 4.593-5.896 g.cm^-3, and V = 0.488425-0.546500 nm³. As evident from the above data, the higher the Se concentration, the larger the crystallographic parameters. The Sb-S-Se binary solid solutions obey Vegard's law.