This paper describes the use of electrical conductivity for measurement the Volatile Fatty Acids (VFA) and bicarbonate concentrations, during anaerobic fermentation process. Two anaerobic continuous processes were studied: the first was a laboratory reactor for hydrogen production from molasses and the second was a pilot process for anaerobic digestion of vinasses producing methane. In case of hydrogen production process, the VFA concentration, but not bicarbonate concentration, can be estimated from the on-line electrical conductivity measurements. In case of methane production process, the bicarbonate concentration, but not the VFA concentration, can be estimated from the simultaneous on-line measurements of pH and electrical conductivity. That shows the interest of electrical conductivity for monitoring the anaerobic digestion processes.