To examine whether end-alveolar breath hydrogen and methane could be used as indicators of fiber digestion in humans, 16 male subjects were fed four fiber-free, complete liquid diets with 0, 30 g (heat processed), 30 g, and 60 g/day soy polysaccharide. Breath hydrogen was measured hourly and breath methane every 4 h on days 6 and 8 of each study period. Feces were collected, homogenized, dried, and analyzed for neutral detergent fiber (NDF). NDF in diets was determined and apparent NDF digestibility calculated. NDF from soy was extensively fermented, >80%, on the fiber-containing diets. No significant relationship was found between breath-gas excretion and fiber digestion, although breath-gas values varied greatly. Breath hydrogen and methane were not significantly different when subjects consumed diets containing 0, 30, or 60 g soy polysaccharide.