Mixed culture fermentations are of interest for the low-cost production of organic acids from complex agricultural waste streams. Models are developed for these processes in order to predict the product spectrum as a function of the environmental process conditions. An important assumption in many existing models for anaerobic mixed culture fermentations is that the NADH/NAD(+) ratio is directly coupled to the dissolved hydrogen partial pressure (pH(2, liquid)). In this study, this assumption was tested experimentally with mixed culture chemostats operated at dilution rates of 0.05 and 0.125 h(-1) for a wide range of calculated dissolved hydrogen partial pressures (0.04-6.8 atm). No correlation was found between pH(2, liquid) and the NADH/NAD(+) ratio. This result, together with thermodynamic calculations, suggests that additional electron carriers such as ferredoxin and formate should be included in models predicting product formation by mixed cultures.