Two identical thermophilic H2 fermenters (R1 and R2) were operated at different pH levels between 4.7 and 5.7. In R1, several unexpected and severe drops in H2 yield inversely proportional to increase in acetate production were experienced at pH 5.5 and 5.7. In contrast, R2 operated at pH 5and 4.7 performed more stable H2 production mainly through butyrate fermentation. Although the H2 partial pressure (>50 kPa) was far above the favorable values, acetate was produced as well as butyrate in all pH levels tested. To determine whether some portion of the acetate is produced through another pathway such as autotrophic synthesis via H2 dependent reduction of CO2 or not, batch dissolved H2 consumption rate tests were performed at pH 5.0, 5.5 and 6. The specific H2 consumption rate was 488(±49) μmol/ at pH 6 and slightly higher than at pH 5and 5.5. The results of continuous and batch experiments revealed that acetogenic H2 consumption is more favorable at pH levels above 5.5 and is one of the reasons of instabilities in dark fermentative H2 production.