The dynamics of the anaerobic conversion of formate in a microbial mixed culture taken from an anaerobic fluidized bed reactor was studied using a new stirred micro reactor equipped with a membrane mass spectrometer. The microreactor with a toroidally shaped bottom and pitched blade turbine and a cylindrical flow guide was thermostated and additionally equipped with a pH electrode and pH control. During fed-batch experiments using formate, the dissolved gases (methane, hydrogen, and carbon dioxide), as well as the acid consumption rates for pH control were monitored continuously. Initially and at the end of each experiment, organic acids were analyzed using ion chromatography (IC). It was found that about 50% of the formate was converted to methane via hydrogen and carbon dioxide, 40% gave methane either directly or via acetate. This was calculated from experiments using (HCO3-)-C-13 pulses and measurement of (CH4)-C-12 and (CH4)-C-13 production rates. About 10% of the formate was converted to lactate, acetate, and propionate, thereby increasing the measured CO2/CH4 production ratio. The nondissociated formic acid was shown to be rate determining. From the relatively high K-S value of 2.5 mmol m(-3), it was concluded that formate cannot play an important role in electron transfer. During dynamic feeding of formate, hydrogen concentration always increased to a maximum before decreasing again. This peak was found to be very discriminative during modeling. From the various models set up, only those with two-stage degradation and double Monod kinetics, both for CO2 and hydrogen, were able to describe the experimental data adequately. Additional discrimination was possible with the IC measurement of organic acids.