The Bureau of Mines has upgraded high-iron domestic chromite concentrates by a carbonyl process. The upgraded chromite concentrates show as much as 10 wt pct higher cr2o3 content and up to a threefold increase in the cr:fe ratio, compared with the starting concentrates. The high-iron chromites were first reduced to convert the iron oxides to the metal, and then treated with carbon monoxide at elevated pressures and low temperatures to convert part of the metallic iron fraction to iron pentacarbonyl. Carbonylation was accelerated by the addition of h2s in small quantities. The iron pentacarbonyl was extricated from the concentrate in the pressure reactor during carbonylation by a CO sweep through a pressure- reducing valve and passed through a decomposer at atmospheric pressure to convert it back to iron metal and carbon monoxide. Carbon monoxide pressure, temperature, promoter trends, and other factors enhancing iron pentacarbonyl formation are presented.