The aim of this work is to compare Molten Carbonate Fuel Cell (MCFC) stack behaviour when square and rectangular cells are used, and to discuss the optimisation of their operating conditions. In particular, experimental data have been collected For a stack with square cells of 0.1-m(2) area to validate a stack simulation model; then, the effects of area increase of up to 0.75 m(2) have been predicted, proving the advantage of working with rectangular cells in order to obtain a better temperature and pressure drop management. Moreover, the operating conditions of a rectangular stack have been analysed and optimised when reformed natural gas, as well as coal gas, are used as fuel. The results presented and discussed have been used to guide design and operation of a rectangular stack constructed by Ansaldo Ricerche (ARI).