We use time-resolved x-ray radiography in order to monitor interdiffusion processes in situ in liquid alloys. We measure temperature and composition dependent interdiffusion coefficients (D{AlCu}) in Al-rich Al-Cu melts. At constant temperature, D{AlCu} only exhibits a weak dependence of the alloy composition. As compared to self-diffusion, interdiffusion is enhanced by a factor of about 3. Our results demonstrate that it is not possible to express interdiffusion in terms of self-diffusion and thermodynamic driving forces.