Orpiment, getchellite and stibnite from Zarshuran deposit, NW Iran, exhibit extensive As-Sb substitution. There are four known minerals along As2S3-Sb2S3 join, i.e. orpiment (As2S3), Wakabayashilite [(As,Sb)20S30], getchellite (AsSbS3) and stibnite (Sb2S3). Extensive mineralogical studies by scanning electron microscope (SEM) and electron probe micro-analyzer (EPMA) indicate that getchellite from Zarshuran exhibits large As/Sb substitution, compared to the synthetic mineral and occurrences in other deposits, which justifies further experimental studies. Natural orpiment with the highest Sb content was recorded in Zarshuran samples. Considering the possible explanation for the incomplete record of solid solution, it appears that rapid variation in fluid chemistry and variability of As and Sb concentration in the hydrothermal solution may have caused extensive solid solution in the As2S3-Sb2S3 system. The recorded solid solution may indicate that temperatures of gold mineralization were less than 250°C which is supported by fluid inclusion data.