The U.S. Bureau of Mines has conducted studies to utilize rapid microwave heating to stress-fracture ore samples. Iron ores containing hematite, magnetite, and goethite were subjected to microwave energy in batch operations at 3 kw and heated to average maximum temperatures between 840 and 940 deg c. Scanning electron microscope photomicrographs verified fracturing along grain boundaries and throughout the gangue matrix. Standard bond grindability tests showed that microwave heating reduced the work index of iron ores by 10 to 24 pct. Using a continuous feed belt in a microwave applicator, samples heated more uniformly and quickly than in batch operations. In a microwave chamber designed to simulate a continuous throughput operation at 3 kw, the grindability of a taconite ore was improved by 13 pct at a bulk temperature of 197 deg c. Because stress cracking occurred at a lower temperature, less energy was consumed, thereby improving the cost effectiveness of microwave-assisted grinding. To further improve the economics of microwave fracturing, preliminary tests were conducted to increase heating rates by using higher microwave powers. To appraise the economic feasibility of microwave-assisted grinding, the beneficial effects in the grinding circuit and in the extractive operation were considered. Factors such as increased throughput, less recycled ore, less wear and maintenance, and improved liberation of minerals were addressed.