Abstract The purpose of this work is to explore the effects of filled materials on NOx emissions in a thermocline storage process of binary molten nitrate (BMN) salts (an eutectic salt mixture with the component of 60%NaNO3-40%KNO3) and determining the most suitable temperature range of BMN salts in the thermocline system. Referring to the national environmental protection standard of PR China, HJ479-2009, the concentrations of NO/NO2 emissions in tail gases from the heated molten salts while contacting with alumina ceramics (Al2O3 ceramics), silicon carbide ceramics (SiC ceramics), magnesium oxide refractory bricks (MgO refractory bricks) and concrete blocks in the magnesium oxide (MgO, 99% purity) crucible were measured in thermal energy storage (TES) process. Moreover, chemical thermodynamic calculations of related reactions that between SiC, Al2O3, SiO2 and nitrate salts were carried out to explain the source of NO/NO2. The experiment results reveal the concentrations of NOx emissions increase with the increasing temperature, and this trend become quite sharp after 500. At 500°C, the concentrations of NOx emission of BMN salts contacting with Al2O3 ceramic, SiC ceramics, MgO refractory bricks, concrete blocks are 160 mg/m3, 211 mg/m3, 210 mg/m3 and 267 mg/m3 respectively. The concentrations of NOx emission of BMN salts contacting with four filled materials are following the sequence: concrete blocks＞SiC ceramics＞MgO refractory bricks＞Al2O3 ceramics.