We describe a precise method for measuring the diffusion coefficient of solutes in liquid metals. A thin layer of a radioactive isotope of the solute is allowed to diffuse in capillaries which can be sectioned using a shear cell device. This method allows a precise measurement of the temperature and of the time duration of the experiment, and avoids any solute segregation when cooling the samples. The experimental results show that there is a negligible direct influence of the valence of a solute on its diffusion coefficient in tin