To determine how soil physical and chemical characteristics affect NH₃ volatilization, we measured NH₃ losses from eight different eastern Canadian soils with various soil clay contents and cation exchange capacities (CEC). Losses were measured from soil mesocosms banded (0.05 m depth) with urea (equivalent of 140 kg N ha^-1) in a dynamic chamber system fitted with an acid trap; with soil properties measured in parallel mesocosms. Regression analysis indicated a negative relationship between 28 d of NH₃ volatilization losses and soil clay content (P < 0.001; R² = 0.978), CEC (P < 0.001; R² = 0.941), and buffer capacity (P = 0.006; R² = 0.772), and positive relations with maximum change in soil pH (P = 0.015; R² = 0.670) and maximum water-extractable NH₄ (P = 0.010; R² = 0.721). A 90% reduction in NH₃ losses occurred when clay content increased from 10% to 20%. Also, the correlation between water-extractable NH₄ and NH₃ loss and a lack of correlation between salt-extractable (1 mol L^-1 KCl) NH₄ and NH₃ loss indicate that NH₄ bound to cation exchange sites does not contribute to the rapid NH₃ volatilization. However, more research on soils of different mineralogy is required to ascertain whether this holds in other regions as well.