A vacuum microwave-assisted dehydration process of orthophosphoric acid for preparation of polyphosphoric acid was studied in comparison with the conventional dehydration in terms of energy efficiency and process rates. The influence of the applied microwave-power input on the process duration and the efficiency of microwave process were examined. The optimal microwave power input provided twofold shortening of the process time compared to conventional heating. At the same time the energy efficiencies remained comparable for both heating modes which was caused by nonoptimal filling ratio of the microwave cavity. The perspectives of the process scale-up in terms of increasing energy efficiency were also discussed. (C) 2011 Elsevier B.V. All rights reserved.