A dual-organism, dual-substrate microbiological reactor was studied using organisms which metabolize either phenol or methanol. The growth characteristics of yield, ratio of oxygen to substrate uptake rates (RO2/Rs), unit rate of oxygen uptake (RO2/Cb), and substrate concentration were evaluated as functions of growth rate in cultures with a single species predominating (SSP) and with a mixture of the two organisms. In both SSP cultures yield and RO2/RS were decreasing functions of growth rate, indicating an increased tendency of the organisms to excrete metabolic by-products at increased growth rates. Ro2/Cg in both cultures was a linear increasing function of growth rate as expected. Both yield and Ro2/R in the mixed culture were lower than in the SSP, indicating an increased tendency toward by-product excretion. The methanol organism indicated the existence of a positive interaction with the phenol organism in mixed culture. The dehydrogenase activity test was unable to predict the relative amounts of phenol and methanol organism in the mixed culture due to excessive variability in SSP cultures and inability to measure phenol organisms activity in mixed culture. The stoichiometry of microbial oxidation of methanol and phenol was found to vary with growth rate but was complicated by possible by-product formation.