A small, stirred, 14.4-mL tank reactor was designed to serve as a measurement cell for short-term investigation of microbial kinetics. A mass spectrometer membrane probe allowed the measurement of the dissolved gases of hydrogen, methane, oxygen, and carbon dioxide. pH was measured by an electrode and controlled by addition of acid or alkali. The highly sensitive measurement of gases with low solubility allowed rapid measurements at very low conversion. In kinetic experiments, a stepwise increase of substrate concentration (method A) and continuous feed of substrate (method B) were used, allowing quick estimation of substrate kinetics. Acetate conversion in mixed culture biofilms from a fluidized bed reactor was investigated. Substrate inhibition was found to be negligible in the concentration range studied. Experiments at various pH values showed that the undissociated acid form was the kinetic determinant. Kinetic parameters for Haldane kinetics of protons were K(S(H)) = 1.3 x 10(-5) mol m(-3) and K(I(H)) = 8.1 x 10(-3) mol m(-3). With free acid (HAc) as the rate determining species, the kinetic parameters for method A were K(S(HAc)) = 0.005 mol m(-3) and K(I(HAc)) = 100 mol m(-3) and for method B were K(S(HAc)) = 0.2 mol m(-3) and K(I(HAc)) = 50 mol m(-3). The maximum biomass activity occurred at around pH 6.5. Acetate was exclusively converted to methane and CO2 at pH > 6.