The kinetic behaviour of immobilized invertase was investigated using calorimetric measurements in an enzyme thermistor device. The hydrolysis of sucrose is an exothermic process with inhibition-like kinetics above 300 mmol/1; glucose gave no measurable heat of reaction and fructose molecules are condensed in an endothermic process. The concentration dependence of the temperature signal of fructose condensation is linear up to 2 mol/1 at an optimal pH of 3.3, and the absolute value of the heat of reaction is only 25% compared with that of the hydrolytic reaction. A higher Km-value and a lower Vmaxare therefore assumed. The reaction heat of hydrolysis of sucrose decreases with increasing product concentration. A shift of the equilibrium by increasing concentrations of both products is accompanied by a competitive condensation reaction in the case of fructose. The stability of invertase in aqueous ethanol solutions was also investigated. It depends on the ethanol concentration and on the time of contact with the solvent. At 30% ethanol no inactivation within 20 min. occurs, whereas at 60% ethanol a linear dependence of the inactivation on the time of contact was found. Above 80% ethanol less than 10% enzymatic activity remained after a five-minute treatment. This irreversible inactivation has to be considered if invertase is applied in water miscible organic solvents.