In the present investigation an electrochemical method for the determination of concentration and diffusion coefficient of ammonia in nitrogen (NH₃/N₂) is proposed by the means of a solid-state amperometric sensor based on yttria-stabilized zirconia (8YSZ) electrolyte. The measured quantity of the studied sensor is limiting current. The experiments are performed for ammonia concentration levels of 0.1–5 vol% in the temperature interval of 375–430 °C. It is shown that the limiting current level is reached at temperature values not higher than 400 °C and it is linearly proportional to the ammonia concentration. This allows obtaining a calibration curve (limiting current vs. ammonia concentration) for the ammonia determination in NH₃/N₂ mixtures. Using the obtained results and diffusion channel parameters, coefficients of ammonia diffusion in nitrogen can also be calculated. It is found that these values are in a good agreement with table values. It is also found that under experimental conditions the ammonia electrochemical oxidation reaction leads to the formation of N₂ rather than N₂O or NO. The obtained results demonstrate the possibility of using such amperometric sensor for the analysis of ammonia-gas mixtures where diluent gas is nitrogen or another inert gas.