Abstract: Process for the production of polyphenyleneoxide, in which:oxygen, or a gas containing molecular oxygen is fed into a liquid reaction medium, solvent of the 2,6-xylenol and of the catalyst cupric salt complexed with an organic base, but nor solvent or substantially not solvent of the polyphenyleneoxide;from time to time samples are collected of the reaction mixture, said samples are treated with a liquid solvent, capable of dissolving polyphenyleneoxide and of stopping the growth of the macromolecules, until a clear and homogeneous solution is obtained;the through time of the so obtained solution is measured in a capillary tube viscometer and the value is determined of the viscosity and/of the molecular weight of polyphenyleneoxide by the comparison with standard values; andon the basis of the so determined value of the viscosity and/or of the polyphenyleneoxide molecular weight interventions are effected on the oxidative polymerizing reaction of 2,6-xylenol.