Abstract: A method for preparing a fluxing oil, having an iodine number ranging from 50 to 200, based on fatty substances of natural origin having been chemically functionalized by oxidation, includes the steps of: i) providing a fatty substance or a mixture of fatty substances of natural origin, ii) subjecting the fatty substance or the mixture of fatty substances of natural origin to at least one transesterification or esterification reaction by at least one alkanol or mono-alcohol, iii) subjecting the compound or mixture of compounds obtained at step ii) to at least one chemical functionalization reaction by oxidation introducing at least one functional group selected from carboxylic acid, epoxy, peroxide, aldehyde, ether, ester, alcohol and ketone groups, and iv) collecting the fluxing oil.