Cyclic and polycyclic aromatic hydrocarbons are among the most widespread contaminants in soils, because they are still used in large amounts. Although poisonous to many organisms, consortia of microorganisms or single species such as Pseudomonas have specialized in the catabolism of these compounds and use them as their sole energy and carbon source. Phenol, together with some of its catabolic intermediates, and the bacterium Pseudomonas putida were chosen for microcalorimetric investigations in a batch and a fermentor/flow system. As many of these aromatics are very poisonous, the concentration limits for growth and metabolism were evaluated optically and calorimetrically, rendering significantly prolonged lag-phases at higher concentrations, but no complete inhibition of metabolism. Under such experimental conditions the calorimetric curves for heat production rate versus time showed reactions following a Michaelis-Menten kinetics in agreement with data from the literature. The calorimetric results proved that phenol is mineralized aerobically to water and carbon dioxide and that an energy balance can be established.