In order to study the effects of the diet fed to animals from which rumen liquor was obtained (inoculum) and culture media pH conditions on in vitro forage dry matter digestibility (IVD), a factorial design experiment was carried out (3 diets and 3 pH conditions). Diets varied in forage to concentrate ratio: 100% forage (F100), 50% forage (F50) and 20% forage (F20). Rumen liquor from animals eating these diets was used to inoculate batch cultures to determine IVD (ANKOM procedure). pH was decreased by means of either: (i) adding barley or grass hay into culture media (10 g/l of media); or (ii) replacing control buffer (bicarbonate) with any of two McIlvaine (MI) solutions, 1000 ml containing 180 (MI180) or 530 (MI530) ml of citric acid. Four forages of different cell wall characteristics were used in digestibility assays. Diet affected pH of the ruminal inoculum (6.8, 6.3 and 4.9 for F100, F50 and F20, respectively). Bicarbonate solution buffered initial pH of culture media (7.0, 6.9 and 6.7 for F100, F50 and F20, respectively). Digestibility was significantly affected (P < 0.001) by the diet fed to donor animals, decreasing as concentrate increased. MI180 and MI530 treatments led to a lower initial pH (6.5 and 5.5 for MI180 and MI530, respectively). The addition of barley or hay lowered final pH (6.8, 6.1 and 6.5 for control, barley and hay treatments, respectively). Barley decreased digestibility of straw and beet pulp, especially with F20. With MI buffers there was a linear decrease in digestibility of grass hay, straw and beet pulp. This decline was greater with buffer MI530 (lower pH) and high proportions of concentrate in the diet. In conclusion rumen pH and the type of diet fed to donor animals have an important effect on forage digestibility, probably due to changes in the ruminal microbial population.