Calorimetry has shown real potential at bench-scale for chem. and biochem. processes. The aim of this work was therefore to scale-up the system by adaptation of a std. com. available 300-L pilot-scale bioreactor. To achieve this, all heat flows entering or leaving the bioreactor were identified and the necessary instrumentation implemented to enable online monitoring and dynamic heat balance estn. Providing that the signals are sufficiently precise, such a heat balance would enable calcn. of the heat released or taken up during an operational (bio)process. Two elec. Wattmeters were developed, the first for detn. of the power consumption by the stirrer motor and the second for detn. of the power released by an internal calibration heater. Expts. were designed to optimize the temp. controller of the bioreactor such that it was sufficiently rapid so as to enable the heat accumulation terms to be neglected. Further calibration expts. were designed to correlate the measured stirring power to frictional heat losses of the stirrer into the reaction mass. This allows the quant. measurement of all background heat flows and the online quant. calcn. of the (bio)process power. Three test fermns. were then performed with B. sphaericus 1593M, a spore-forming bacterium pathogenic to mosquitoes. A first batch culture was performed on a complex medium, to enable optimization of the calorimeter system. A second batch culture, on defined medium contg. three carbon sources, was used to show the fast, accurate response of the heat signal and the ability to perfectly monitor the different growth phases assocd. with growth on mixed substrates, in particular when carbon sources became depleted. A max. heat output of 1100 W was measured at the end of the log-phase. A fed-batch culture on the same defined medium was then carried out with the feed rate controlled as a function of the calorimeter signal. A max. heat output of 2250 W was measured at the end of the first log-phase. This work demonstrates that real-time quant. calorimetry is not only possible at pilot-scale, but could be readily applied at even larger scales. The technique requires simple, readily available devices for detn. of the few necessary heat flows, making it a robust, cost-effective technique for process development and routine monitoring and control of prodn. processes. [on SciFinder (R)]