A x-ray radioscopy technique for measuring in situ chemical diffusion coefficients in metallic melts is presented. The long-capillary diffusion measurement method is combined with imaging techniques using microfocus tubes and flat panel detectors in order to visualize and quantitatively analyze diffusive mixing of two melts of different chemical composition. The interdiffusion coefficient as function of temperature and time is obtained by applying Fick's diffusion laws. Tracking the time dependence of the mean square penetration depth of the mixing process allows to detect changes in the mass transport caused by convective flow. The possibility to sort out convective mass transport contributions from analysis enhances significantly the accuracy compared to the conventional long-capillary diffusion measurement method with postmortem analysis. The performance of this novel diffusion measurement method with x-ray radiography technique is demonstrated by a diffusion experiment in an Al-Ni melt.