The effect of pH on dark fermentative H2 production at 55°C was studied using three different inocula namely windrow yard waste compost (G), anaerobic organic waste compost (H) and activated sludge (A) and 2 g/L glucose as substrate. The sequential batch experiments were performed by controlling the pH at 5, 5.5 and 6. The highest molar H2 yields were found to be 1.7 at pH 5 for G and H and 1.6 at pH 5.5 for A, while severe drops in the subsequent runs were associated with lactate fermentation. At pH 5.5 in G, despite the butyrate formation ceased, lower but more stable H2 yields of about 1.1 were obtained through acetate-ethanol fermentation. Besides, the clostridial thermopiles prevalent at pH 5 were substituted by Thermoanaerobacterium-related species at pH 5.5 and 6. The findings suggested that for a stable H2 yield through acetate-butyrate fermentation at 55°C with compost inoculum G or H, the pH has to be controlled between pH 5 and 5.5. This is one of the few papers discussing the effects of pH on H2 production through thermophilic mixed fermentation of glucose by using three different natural mixed culture inocula.