A  PEM type Microbial Fuel Cell was designed and constructed. This cell consisted of four pairs of chambers with a volume of 0.5 L each one of them, separated by a Nafion®117 membrane. The cell was monitored during 122 days, using synthetic wastewater as carbon source. The reduction of cation exchange capacity in the membrane was evaluated in two phases: 1 and 2, the first one at 43 days and the second one at 79 days. The cationic exchange coefficients obtained were 2.03 x 10^-3 and 1.25 x 10^-3 meq g^-1 polymer, for phase 1 and 2 respectively, indicating a decrease in the cation exchange properties in the membrane. The maximum obtained power densities were 325 and 97 mW.m^-3, respectively. The reduction of power density at phase 2 was attributed to a decrease in the cation-exchange capacity by iron fouling and a formation of a biofouling layer in the membrane.