This study determined volatile fatty acids (VFA) from ruminal microbial fermentation in relation to oxidation reduction potential (ORP) and electricity generation from straw in microbial fuel cells. The effect of substrate straw to grain ratio on gas production, VFA, pH, and ORP during rumen microbial incubation with or without the presence of protozoa was evaluated. Increasing straw relative to grain in the substrate mixture resulted in lower gas levels, VFA, and ORP, but a higher pH. The ratio of acetate to propionate increased when more straw was included in the mixture. When straw was the sole substrate, the ratio of acetate to propionate decreased in response to increasing straw addition to the fermentation tubes. A correlation analysis showed that ORP values were inversely correlated with the ratio of acetate to propionate, with a coefficient of -0.712. When straw was added to MFCs, the VFA increased almost two-fold after inoculation with ruminal fluid. Maximum power density reached 0.00309 mW/m(2) following a second transfer of spent fermentation fluid to fresh MFCs. Acetate was the major VFA in the straw-fed MFC, followed by propionate and butyrate. Their concentrations were negatively associated with voltage output. However, the acetate to propionate ratio was positively linked to electricity production. The results illustrate that the VFA profile during ruminal microbial fermentation could be influential on ORP and electricity generation in MFCs. The results provide evidence that native fiber and straw could be utilized for electricity production by ruminal microbial MFCs. (C) 2011 Elsevier Ltd. All rights reserved.