A new microcalorimetric titration/perfusion vessel has been developed with special reference to needs in thermodynamic and analytical investigations of complex processes. The vessel, which is part of a twin heat conduction microcalorimeter, is equipped with electrodes for determination of pH and oxygen concentration and with light guides forming part of a died array spectrophotometer. The dynamic properties of the instrument has been investigated. The calorimeter and its analytical sensors has been tested in titration experiments and in bacterial growth experiments.