Aromatic compounds (e.g. phenol) are known to be assimilated via two pathways initiated by the cleavage of the ring structure in the intradiol (ortho) or extradiol (metal position of catechol intermediate. There is some biochemical and physiological evidence that the ortho pathway is energetically more efficient than the meta pathway, i.e. the former results in higher growth yield than the latter. Alcaligenes eutrophus JMP 134 (now, Ralstonia eutropha) possesses both possibilities. As the growth efficiency corresponds to the heat production, calorimetric measurements can help determine (i) which of the two pathways is activated under what conditions and (ii) whether or not the upper limit of carbon conversion into biomass is reached. To answer these questions, a feeding system for a fermentation calorimeter was constructed which allowed to adjust dilution rates, the concentration of substrates and the composition of feed as a function of time. It was shown that during chemistatic growth A. eutrophus uses the ortho pathway up to a dilution rate of 0.25 h(-1). At this point the rate of heat production changed, indicating a shift to the meta pathway. Phenol can be used as a sole source of carbon and energy for growth. But the energy, which can be made available, does not reach to assimilate the phenol-carbon consumed. This means that phenol is deficit in energy. Formate can balance the carbon/energy ratio and provides energy to increase phenol-carbon conversion. By adding formate, the yield coefficient grew from 0.56 g dry mass/g phenol to 1.03 g/g, and in the end it was found that the net gain of energy from the formate is not constant.