This European Standard specifies the standard reference method (SRM) for the determination of the sulphuric oxide SO2 in flue gases emitted to the atmosphere from ducts and stacks. It is based on a sampling system and two analytical principles: ion chromatography and the Thorin method. This European Standard specifies the performance characteristics to be determined and the performance criteria to be fulfilled by measuring systems based on the measurement method. It applies to periodic monitoring and to the calibration or control of automatic measuring systems (AMS) permanently installed on a stack, for regulatory or other purposes. This European Standard specifies criteria for demonstration of equivalence of an alternative method to the SRM by application of EN 14793:2017. This European Standard has been validated during field tests on waste incineration, co-incineration and large combustion installations. It has been validated for sampling periods of 30 min in the range of 0,5 mg/m3 to 2 000 mg/m3 of SO2 for an ion-chromatography variant and 5 mg/m3 to 2 000 mg/m3 of SO2 for the Thorin method according to emission limit values laid down in the Directive 2010/75/EU. NOTE 1 Emission limit values for SO2 laid down in the Directive 2010/75/EU are in the range of 30 mg/m3 to 800 mg/m3. The emission limit values of EU Directives are expressed in units of mg/m3 of SO2 on dry basis and at standard conditions of 273 K and 101,3 kPa. NOTE 2 The characteristics of installations, the conditions during field tests and the values of repeatability and reproducibility in the field are given in Annex A.