The solubility and the activity of aluminum chloride hexahydrate, alcl3.6H2o, in aqueous solutions were determined as a part of the Bureau of Mines, U.S. Department of the Interior, research to improve mineral processing technology for recovering alumina from low-grade domestic resources. The solubility of alcl3 in pure water was found to be 31.09 and 31.77 wt-pct at 25 deg and 85 deg c, respectively, and decreased drastically to 10 and 0.1 wt-pct when the respective amounts of 20.3 and 37 wt-pct hydrochloric acid (hcl) were added in solution. The equilibrium constant for the solubility of hexahydrate, activities, and activity coefficients of alcl3 and h2o were calculated and compared with the results from the available methods of estimation. The results were shown to be useful in leaching nonbauxitic ores for extraction of alumina by changing the solubilities of component salts upon addition of hcl. Simultaneous solubilities of alcl3, fecl3 kcl, and nacl in solutions of hcl-h2o were also determined and their effects on the purity of alcl3.6H2o crystallized from mixed salt solutions are briefly discussed.