Partial load efficiencies of a natural gas fuelled MCFC/GT system are calculated; the efficiencies of four systems are compared. A constant pressure air compressor is applied in system cases 1 and 2, whereas a pressure swing air compressor is provided in system cases 3 and 4. A gas cooler is integrated in the cathode gas recycling line of cases 2-4, and an anode recycling with sub-reformer is provided in case 4. The cathode pressure loss in the MCFC stack is kept below 3 kPa during the calculation procedure to avoid a leakage of cathode gas. The range of the power load is limited to 50-100% in the constant operating pressure system (cases I and 2), mainly because of the limited cathode gas pressure loss of 3 kPa. The range of the power load is enlarged to 20-100% in cases 3 and 4 by combining the pressure swing operation with gas cooling in the cathode recycling line. In system cases 3 and 4, the efficiency at the lowest load operation (approx. 20-30% load) remains over 35% HHV-CH4, whereas the maximum efficiency is calculated to be 53% HHV-CH4 in middle load operation; the efficiency of case 4 at 100% load is estimated to be 50% HHV-CH4. The combination of the pressure swing operation and gas cooling in the cathode recycling line offers a high efficiency of the MCFC system in a wide range of loads. (C) 2004 Elsevier B.V. All rights reserved.