The invention relates to a process for the preparation of azelaic acid or alkyl azelate starting from mono-unsaturated 9-octadecenedioic acid or its corresponding alkylester characterized in that the process comprises at least the following step: •reacting the mono-unsaturated 9-octadecene dioic acid or its corresponding alkylester with hydrogen peroxide in the presence of an organic carboxylic acid other than 9-octadecene dioic acid and a suitable catalyst to effect cleavage of the double bond in the mono-unsaturated 9-octadecene dioic acid or its corresponding alkylester. The invention further relates to the azelaic acid or alkyl azelate obtainable by the process according to the invention and to the use of azelaic acid or alkyl azelate or monomers derived there from for the preparation of a polymer, especially a polyamide.