Achieving high homogeneity of AdBlue distribution dispensed to the exhaust system is a major design challenge. Simulation of the distribution of ammonia on the surface of the selective catalytic reduction (SCR) reactor often proves inadequate and the best solution for optimisation of the design is to perform measurements of the heterogeneity of the distribution. The measurements are made by means of an automatic sampling probe that allows mapping of any grid of points on the downstream face of the SCR reactor. Measurements of the concentrations of ammonia and nitrogen oxides are performed, on the basis of which the heterogeneity of their distribution can be assessed. It is also possible to measure the remaining gaseous components of the exhaust gas. Results obtained in this way clearly identify locations that are too rich and too poor in ammonia, thus clearly defining the necessary corrections.