A test portion is dissolved in ethanol and the solution is acidified. The alkane monosulfonic acids are separated from the alkane disulfonates and sulfate ions by extraction with light petroleum. The light petroleum phase is evaporated and the residue is solved in ethanol. Traces of alkali are removed by means of a cation-exchange column. The eluate is neutralized with carbonate-free sodium hydroxide solution. The sodium alkane monosulfonates so formed are dried and weighed. The mean relative molecular mass of the alkanemonosulfonates and the alkanemonosulfonate content are calculated.