Process for operating a plant for preparing an alkanesulfonic acid, wherein said alkanesulfonic acid is prepared by oxidation of an alkylmercaptan, dialkydisulfide and/or dialkylpolysulfide, characterized in that the oxidation is performed in the presence of an at least stoichiometric amount of an oxoacid of nitrogen relative to the amount of said alkylmercaptan, dialkyldisulfide and/or dialkylpolysulfide during the start-up phase of the plant, comprising the steps i) providing an oxoacid of nitrogen in a reactor, ii) feeding an alkylmercaptan, dialkydisulfide and/or dialkylpolysulfide containing stream into the oxoacid of nitrogen of step i) to give an alkanesulfonic acid containing product mixture, and iii) repeating step ii) until the available inner volume of the reactor is filled completely with the alkanesulfonic acid containing product mixture obtained in step ii), iv) feeding additional oxoacid of nitrogen into the reactor, when the amount of oxoacid of nitrogen provided in step i) is not enough to fill the available inner volume of the reactor with the product mixture obtained in step ii), v) feeding an oxygen containing fluid stream into the reactor, when the available inner volume of the reactor is completely filled with the product mixture obtained in the step ii) and/or in the step iii), vi) removing at least part of the alkanesulfonic acid containing product mixture from the reactor, and vii) feeding an alkylmercaptan, dialkyldisulfide and/or dialkylpolysulfide containing stream into the reactor to yield a reaction mixture, wherein said alkylmercaptan, dialkyldisulfide and/or dialkylpolysulfide is identical with the alkylmercaptan, dialkyldisulfide and/or dialkylpolysulfide of any of the preceding steps, wherein the volume of the alkylmercaptan, dialkyldisulfide and/or dialkylpolysulfide containing stream, fed into the reactor in step vii), equals the volume of the product mixture removed from the reactor in step vi). i) providing an oxoacid of nitrogen in a reactor, ii) feeding an alkylmercaptan, dialkydisulfide and/or dialkylpolysulfide containing stream into the oxoacid of nitrogen of step i) to give an alkanesulfonic acid containing product mixture, and iii) repeating step ii) until the available inner volume of the reactor is filled completely with the alkanesulfonic acid containing product mixture obtained in step ii), iv) feeding additional oxoacid of nitrogen into the reactor, when the amount of oxoacid of nitrogen provided in step i) is not enough to fill the available inner volume of the reactor with the product mixture obtained in step ii), v) feeding an oxygen containing fluid stream into the reactor, when the available inner volume of the reactor is completely filled with the product mixture obtained in the step ii) and/or in the step iii), vi) removing at least part of the alkanesulfonic acid containing product mixture from the reactor, and vii) feeding an alkylmercaptan, dialkyldisulfide and/or dialkylpolysulfide containing stream into the reactor to yield a reaction mixture, wherein said alkylmercaptan, dialkyldisulfide and/or dialkylpolysulfide is identical with the alkylmercaptan, dialkyldisulfide and/or dialkylpolysulfide of any of the preceding steps, wherein the volume of the alkylmercaptan, dialkyldisulfide and/or dialkylpolysulfide containing stream, fed into the reactor in step vii), equals the volume of the product mixture removed from the reactor in step vi). <EOS>