A 2 x 4 factorial experiment was carried out with eight SICV-cannulated pigs of 30 kg initial BW. Dietary buffering capacity (632 vs 578 meq/kg in Diets H and L, respectively) in combination with supplementary organic acids (formic acid, fumaric acid or n-butyric acid in acid-equivalent doses of 300 mmol/kg) were evaluated for their effects on the apparent ileal digestibility (AID) of protein and amino acids. In the presence of these organic acids in Diet H ("practical"), the AID of crude protein and several amino acids increased by 2.9 to 5.9%-units (P < 0.05). This increase was linear and positively correlated with increasing dietary acidity. Feeding Diet L without organic acids resulted in an increase of urinary acidity by 1.6 pH-units, and thereby, ammonia emission from manure was 44% lower.