Abstract: For quantitative analysis of gas volumes, specifically combustion exhaust gases, by means of emission or absorption spectrometry in the ultraviolet, visible and infrared spectral range, geometrically defined and reproducibly adjustable observation planes are oriented perpendicular to the longitudinal axis of an exhaust stream. In a first series of measurements a number of m spectral measurements is performed, in such a way that the optical axis of a spectrometer is always located in the respective observation level but is shifted in a parallel direction by a first distance from one measurement to the next. In a second series of measurements n measurements are performed, in such a way that the optical axis is again located in the observation plane and is shifted in a parallel direction by a second distance from each measurement to the next. The (m+n) measurements produce two orthogonal sets of line of sights, which form a grid with (m·n) intersecting volumes.