Abstract: An apparatus and a method for determining the composition of a protein-containing specimen has a combustion chamber for the combustion of the specimen, an exhaust gas line connected to a combustion chamber heated by a heating device, a combustion control device, an oxygen inflow line opening into the combustion chamber with an oxygen inflow valve, which can be operated by means of the control device, and an analyser connected to the combustion chamber. An oxygen sensor connected to the control device determines the oxygen content in the combustion gases. The oxygen content is determined in time intervals or continuously during combustion. If a pre-specifiable concentration of oxygen is not achieved, a definable quantity of oxygen is injected into the combustion chamber by the oxygen inflow valve. After complete combustion of the specimen, the combustion gases are delivered to the analyser to determine nitrogen, carbon dioxide, sulphur dioxide and/or water content.