Enzymatic urea hydrolysis has been investigated calorimetrically as a function of urea concentration, the buffer system and the pH value. It is shown that the reaction follows a complex mechanism. Unambiguous conclusions on the reaction sequences are made possible by extensive thermodynamic calculations. The most important conclusion from the experimental results and thermodynamic calculations is the absolute necessity of knowledge of the experimental conditions, i.e. urea concentration, pH value and buffer system, for the meaningful discussion of calorimetric data. The molar reaction enthalpy of the enzymatic partial reaction is determined to be Delta(R)H(1) =(-26.5 +/- 1.5) kJ per mole urea. Thermokinetic evaluation by means of a simple consecutive reaction model allows the velocity constants to be calculated. For the enzymatic partial reaction, parameters K-m and upsilon(max) are determined in phosphate buffer and HEPES buffer.