Abstract: A composition comprising a potassium salt of N?-hydroxy-N-cyclohexyldiazenium oxide (KHDO) and a diluent is useful for killing fungi. Such a composition but which additionally includes at least one of certain other fungicidally active components is useful for combating microorganisms in general. Such other fungicidally active component is selected from: alcohols, isothiazolones, activated halogen compounds, formaldehyde release compounds, phenolic compounds, aldehydes, acids and esters, biphenyls, urea derivatives, O-acetals, O-formals, N-acetals, N-formals, benzamidines, phthalimides, pyridine derivatives, quaternary ammonium and phosphonium compounds, amines, amphoteric compounds, dithiocarbamates, compounds containing active oxygen and mixtures of any of these.