Two studies were conducted using piglets fitted with a simple T-cannula at the distal ileum to determine the effect of fumaric acid supplementation and different buffering capacities on ileal digestibilities and on the level of microbial metabolites in ileal digesta. Supplementation of fumaric acid to a diet with a low buffering capacity (Experiment 1) improved the ileal digestibilities of energy, crude protein and most of the amino acids (P < 0.05). The decrease in microbial metabolites with increasing level of fumaric acid supplementation indicates a lower microbial activity in the upper digestive tract. Supplementation of fumaric acid to a diet with a high buffering capacity (Experiment 2) showed a trend (P > 0.05) towards an increase in ileal nutrient digestibilities. Digestibility values in experiment 2 were 2–10 percentage units lower compared to the corresponding digestibility values in Experiment 1.