System and method for determining the ethanol content in fuel

There are provided, which is part of a mixture of fuel and air, which is burned in an internal combustion engine, a system and method for determining an ethanol content of a fuel. A printing characteristic of a fuel rail is monitored during the operation of the internal combustion engine. An effective bulk modulus of the fuel and / or a pressure disturbance signature is or are determined on the basis of the printing property. The ethanol content is determined on the basis of the effective compression module and / or the pressure fault signature.