Important engineering aspects related to the production of polyoxadiazole (POD), from the polycondensation of 4'4'-diphenyl-etherdicarboxylic acid (DPE), and hydrazine sulphate (HS), in poly(phosphoric acid) (PPA), such as reproducibility of final polymer properties and sensitivity to initial feed charge, were investigated. When the reaction variables are not tightly controlled, large variations of molecular weight and macromolecular structure of polymer materials synthesized at similar reaction conditions may be observed. These experimental fluctuations pose a challenge for the design and scale-up of POD synthesis. It seems that the two most fundamental sources of experimental perturbation are the degradation of the polymer chains during the purification step and the composition fluctuations during feeding of the reaction system. Despite of that, polymer samples without residual hydrazide group and with high molecular weights (higher than 200 kDa) can always be obtained when the final polymer sample is shredded and homogenized, the pH of the suspension that contains the precipitated polymer is rigorously controlled and the local fluctuations of the initial feed composition are minimized through the increase of the reaction scale. This indicates that the final purification of the POD is of fundamental importance for proper control of the final polymer properties. (C) 2004 Elsevier Ltd. All rights reserved.