100 cm2 molten carbonate fuel cells (MCFC) was used for testing the fuel and oxidant composition influence on MCFC performance as a temperature function. The fuel composition varies over the cell surface, and throughout the stack, because inlet fuel is rich in H2 (and CO2), while the exhaust stream is depleted. The cell performance is usually not uniform over the entire surface. It is thus necessary to examine the cell performance as a function of fuel utilization. In a laboratory-sized cell, this is usually difficult to do. An alternative approach is to investigate performance as a fuel composition function. Based on the obtained experimental data for different compositions, the MCFC mathematical model was calibrated. The new approach for modeling the voltage was used. Electrochemical, thermal, electrical and flow parameters are collected in the 0-D mathematical model. A validation process for various experimental data was made and adequate results are shown.