Abstract: A fluorimetric method for the quantitative determination of SO.sub.2 in gas provides a stable measuring base. It consists in lowering the background values in terms of SO.sub.2 concentrations to about 2/10 of one ppm by increasing the oxygen concentration in a reference gas to at least 2%. This is effected by adding to this reference gas an O.sub.2 -containing stream or an ozone-containing stream. When the ozone-oxygen mixture is used the increase in O.sub.2 concentration need not be higher than 0.5-1.0% (instead of at least 2%) to achieve the same background SO.sub.2 reduction (see FIG. 4).