Nafion membranes modified with vinylphosphonic acid (VPA) and 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPSA) polymers have been obtained and characterized. The membranes were used in methanol fuel cells, Current-voltage profiles were obtained in a Globe Tech GT 60 station adapted to work in a system supplied with methanol. The measurements were conducted in a constant current or constant voltage mode with control of temperature of the cell, fuel and oxygen and control of flow and pressure of methanol and oxygen. It was found that the insertion of VPA into a Nafion membrane increases the power density of the fuel cell,