The guideline is valid for multi-gas sensors for differentiating, recognizing and quantitative determining of mixtures of gaseous materials. It is a manual for manufacturers, offerers and users and supplies informations and explanations for the employment of multi-gas sensors. Terms and characteristics of the multi-gas sensor systems are defined and the structure of multi-gas sensors is described. For the application-oriented employment a classification is defined of multi-gas sensors. Grades to the evaluation of the abilities simplify the selection of a suitable sensor and facilitate the evaluation of multi-gas sensors.