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Abstract. The poultry industry has successfully used aluminum sulfate (alum) as a litter amendment to reduce NH₃ emissions from poultry barns but the technology has not been utilized in the beef industry. The objective of this study was to measure NH₃ from feedlot surface material (FSM) containing alum. A 33-day lab-scale study was conducted using pans containing FSM and either 0, 2.5, 5, or 10% alum. The pH of the FSM was significantly lower (P < 0.01) when treated with 2.5, 5, and 10% alum as opposed to the 0% alum treatment. Ammonia emissions were immediately reduced when FSM was treated with alum. No NH₃ volatilization occurred below a pH of 6.5. A second study was conducted at the feedlot. Six feedlot pens that recently had cattle removed were used. In each pen, three areas were treated with 5% alum and three areas were left untreated. There was a significant decrease (P < 0.01) in pH in the areas of the pens where alum was added compared to those that did not receive alum. Ammonia concentrations were lower (P < 0.05) for the areas that received alum compared to those that did not receive alum for the first 7 days of the study, but were higher for the alum-treated locations on Day 10, and similar between the two areas for the remainder of the study. We concluded that 5% alum may reduce NH₃ emissions from beef feedlots, although frequent reapplication may be needed to maintain the suppression in NH₃.