Acetonitrile is an important solvent and intermediate in the chemical industry and is nowadays available as a by-product from the manufacture of acrylonitrile. To develop a more industrial independent and sustainable production route based on renewable sources, a direct synthesis pathway of acetonitrile from acetic acid or ammonium acetate in sub- and super-critical water was investigated. The experiments were carried out in a high pressure plant. With a two-reactor-concept and without addition of a catalyst, acetonitrile could be produced in a nearly quantitative yield.