In this study, the effect of reduced hydrogen partial pressure (P-H2) on the generation of carboxylic acids from acidogenic fermentation of glucose was investigated. Three strategies were applied to reduce P-H2: headspace removal (T1), CO2 sparging (T2) and H-2:CO2 (80:20) sparging (T3). Results showed that the production of carboxylic acids in T1-T3 were 10.21, 11.64 and 12.71 g/L, respectively, which were 1.04, 1.19 and 1.30-fold of that in the control (T4). The composition of carboxylic acids changed significantly in T3 with enhancement of homoacetogenesis, as more acetate and butyrate were produced comparing to the control. In addition, decreasing P-H2 led to more carbon flow to carboxylic acids. Species of Clostridium became dominant in treatment T3, resulting in the shift of metabolic pathways. This study demonstrated that decreasing P-H2 could increase the production of carboxylic acids, especially under the strategy of enhancing homoacetogenesis.