The diffusion rate of magnesium in tungstened nickel was measured at temperatures between 970 and 1330 K. The method consists of measuring the species and number of atoms collected on a collector in front of the evaporating surface in a well‐defined geometrical arrangement under ultrahigh‐vacuum conditions. The collected material is analyzed by means of AES. Experiments on the diffusion of magnesium in tungstened nickel yield an activation energy of 57 kcal/mole. The vapor collect method shows great potentialities for the measurement of evaporation rates and its dependence on surface contaminants.