The main message of this paper is the description of an analyzing technique about the measuring of the energy budget in pressure vessels on a laboratory scale. To demonstrate this technique two cases are presented. The first series deals with the oxidation of cellulose and the second series with the co-treatment of jarosite and sludge. Jarosite is a waste product of zinc refining factories using hydrometallurgical processing routes. The energy budget is always a parameter in process design. A common way to obtain the net energy transfer of reactions occurring at atmospheric pressure is to obtain the differential thermal analyses (DTA) signal by using a joule measuring device'. Obtaining the reaction energy at higher pressures is more complicated. Indirect information was obtained by measuring the temperature rise produced by the ongoing reaction in an autoclave during heating, compared to the temperature profile of the same autoclave during a blank experiment, This method is called here 'Sequential Differential Thermal Analyses', (S-DTA). The disadvantage of such a system is the energy loss to the environment, since the autoclave is not perfectly isolated. It has been shown that for limited energy increases this effect is negligible, and for stronger exothermic reactions the autoclave should be used with smaller quantities of reagents. The wet oxidation of cellulose with varying amounts of oxygen has been used to demonstrate the possibilities and the accuracy of this method.