In the present work a high temperature oscillating cup viscometer has been used to measure the viscosities of liquid binary Al-Cu alloys. The dependence of viscosity on temperature is well described by the Arrhenius law. For constant temperature, the viscosity as a function of copper concentration exhibits a maximum at a mole fraction x (Cu) = 0.7. This might be due to a pronounced chemical short range order in the liquid phase at this composition. As the comparison of existing phenomenological models describing viscosity as a function of composition to the experimental data is unsatisfactory, a new model for the viscosity has been developed within this work based only on a few assumptions and using the enthalpy of mixing as input parameter which is easily accessible. The agreement between model calculation and experimental data is excellent.