Four Jersey cows, fitted with ruminal and duodenal cannulas, were used to assess the specific effects of changes in ruminal pH that were induced by barley addition to a hay diet on ruminal fiber digestion. Cows received successively 100% unchopped cocksfoot hay (diet H), 65% hay and 35% pelleted ground barley (diet HB), and HB plus a continuous intraruminal infusion of bicarbonate salt solution (diet HBB). Cows were limit‐fed once daily at 7 kg of DM day‐1. The ruminal fractional passage rate of the liquid and hay was respectively estimated from a single administration of Cr‐EDTA and Eu labeled on hay fiber. Duodenal fiber flow was estimated both by the double marker method (Yb‐acetate/Cr‐EDTA) and by multiplying the fractional passage rate of Eu‐labeled hay by ruminal pool size. Barley supplement decreased ruminal NDF and ADF digestion compared with diet H, but passage rates of the solid and fluid fractions in the rumen were not affected. The depressive effect of barley supplement on in situ degradability of hay NDF and ADF decreased the degradation rates and increased the 72‐h undegradable fraction. Compared with diet HB, ruminal NDF and ADF digestion was improved with buffer supplementation but was less than that of diet H, eventhough ruminal pH was similar to that of diet H. Passage rates of the fluid and particle fractions were depressed and unaffected, respectively, with buffer addition. Higher in situ degradability of hay with the buffer supplement resulted in an increase in the 72‐h undegradable fraction, but the fractional degradation rate was unaffected.