A process for the addition of an azide function to an organic compound in which process a mixture is prepared by adding an epoxide-derivative of the organic compound and an alkali metal azide salt to a solvent is described. The mixture is heated to a reaction temperature at which the epoxide-derivative and the azide can react to form an azide derivative of the organic compound. An amount, near equimolar to the epoxide derivative, of a (1-6C)alkyl-(2-4C)carboxylic acid ester having a boiling point above the reaction temperature is added to the mixture before and/or during the reaction.