The present invention concerns a method in connection with the supplying of a first additive for treating an exhaust gas stream resulting from combustion in a combustion engine, wherein said first additive is supplied to said exhaust gas stream, and wherein said first additive is utilized for the reduction of at least a first substance (such as NOx) present in said exhaust gas stream, wherein the method comprises: estimation of a reduction of said first substance based on a first measurement of a presence of said first substance in said exhaust gas stream downstream of said supply of said first additive, and a second measurement of a presence of said first substance in said exhaust gas stream upstream of said supply of additive, comparison of said estimated reduction to a first reduction, and correction of the estimation of said second measurement of said first substance based on said comparison.