The two calorespirometric ratios, the ratio of metabolic heat rate to the rate of CO2 production and the ratio of metabolic heat rate to the rate of O-2 uptake (R-q/R-CO2 and R-q/R-O2, respectively), provide different information about the activities of metabolic pathways. In a steady state system, R-q/R-CO2 depends only on the oxidation state of the substrate and R-q/R-O2 equals Thornton's constant or the oxycaloric equivalent. In a growing or developing system, the measured ratio R-q/R-O2 differs from the oxycaloric equivalent only if reactions that do not consume oxygen and have nonzero DeltaH are present. Relative rates of aerobic and anaerobic (with DeltaH not equal 0) reactions can thus be calculated from the measured R-q/R-O2, but the substrate carbon conversion efficiency cannot. The difference between the R-q/R-CO2 ratio predicted from Thornton's rule and the actual measured ratio contains information on the rates of anaerobic reactions with DeltaH approximate to 0. The latter ratio thus allows partitioning of the CO2 production rate between oxidative catabolism and anabolic reactions with DeltaH approximate to 0. This partitioning allows calculation of substrate carbon conversion efficiency and rates of growth or development. (C) 2004 Elsevier B.V. All rights reserved.