This document specifies the general principles, including testing procedures and requirements, for the classification of performance of low-cost sensor systems for the monitoring of gaseous compounds in ambient air at fixed sites. The classification of sensor systems includes tests that are performed under prescribed laboratory and field conditions. The procedure described is applicable to the determination of the mass concentration of air pollutants. The pollutants that are considered in this document are the gaseous pollutants regulated under Directive 2008/50/EC (O3, NO/NO2/NOx, CO, SO2 and benzene) in the range of concentrations expected in ambient air. This document provides a classification that is consistent with the requirements for indicative measurements and objective estimation defined in Directive 2008/50/EC. In addition, it provides a classification for applications (non-regulatory measurements) that require more relaxed performance criteria. This document applies to sensor systems used as individual systems. It does not apply to sensor systems as part of a sensor network. However, for some applications (e. g. in cities) sensor systems are deployed as part of a sensor network. Annex A gives information on the use of sensor systems as nodes in a sensor network. This document gives guidance on the testing of CO2 sensor systems in Annex B since, although not listed in Directive 2008/50/EC, CO2 is an interesting indicator as proxy for activities involving combustion processes or for CO2 evaporation from soil or water.