The double compression expansion engine (DCEE) is a promising concept for high engine efficiency while fulfilling the most stringent European and US emission legislation. The complete thermodynamic cycle of the engine is split among several cylinders. Combustion of fuel occurs in the combustion cylinder and in the expansion cylinder the exhaust gases are over expanded to obtain high efficiency. A high-pressure tank is installed between these two cylinders for after-treatment purposes. One proposal is to utilize thermal reduction of nitrogen oxides (NOx) in the high-pressure tank as exhaust temperatures can be sufficiently high (above 700 °C) for the selective non-catalytic reduction (SNCR) reactions to occur. The exhaust gas residence time at these elevated exhaust temperatures is also long enough for the chemical reactions, as the volume of the high-pressure tank is substantially larger than the volume of the combustion cylinders. In this paper a single-cylinder D13 engine was run together with a 30 l high-pressure tank, with and without a diesel oxidation catalyst (DOC). AUS 32 and an ammonia-water solution (AAS 25) are injected before the high-pressure tank at different exhaust temperatures to study the thermal reduction of NOx produced from the combustion and the impact of the DOC. Additionally, the normalized stoichiometric ratio (NSR) was swept to evaluate the maximum NOx reduction potential of SNCR. Experimental results showed that very high NOx conversion efficiencies could be achieved for both AUS 32 and AAS 25. NOx conversion efficiencies of 80 % were obtained for NSR = 3. At stoichiometric NOx reductant dosing (NSR = 1), 40 % of nitrogen oxides could be reduced thermally. Presence of a DOC would decrease the efficiency of the thermal reduction as it oxidizes ammonia. At exhaust gas temperatures below 400°C, platinum in the DOC reduced NOx with a maximum conversion efficiency of 31 % at 350°C.