The construction of a new type of immobilized enzyme sensor for the determination of chemical species is described. The sensitive part of a thermistor is coated with an artificial enzyme membrane obtained by the cross-linking of the enzyme together with an inert protein, using glutaraldehyde as a bi-functional reagent. The enthalpy change during the enzymatic reaction induces a production or absorption of calories which can be taken as a reaction product and depends then upon the reaction parameters (Km, Vm, [S]). This type of enzyme-bound thermistor is used for the determination of hydrogen peroxide, glucose, and urea. The pH and temperature dependence of the hydrolysis of urea by urease given by this sensor are compared to the results previously obtained by other methods.