Abstract: High molecular weight polyethylene terephthalate (PET) is obtained by solid state polycondensation of low molecular weight polymer granulate or chips. The present invention relates to a moving bed reactor wherein the crystallized, low molecular weight polymer granulate or chips move by gravity flow from the top to the bottom of a cylindrical, vertical reactor. By means of an automatic sampling system, samples of the polymer chips are taken from different points in the moving bed and analyzed to continuously establish the intrinsic viscosity, i.e. molecular weight, of the chips. A countercurrent flow of cold and/or heated gas and the chip residence time are then adjusted in response to the viscosity profile in the reactor in order to control and achieve a desired chip quality.