Interdiffusion measurements were calculated using the Boltzmann-Mattano analysis of concentration profiles and by following the thickness and resistance of the diffusion cell as a function of time. The results for the equimolar solid solution Ag0.5Cu0.5 I were coincident for the three methods, the values of the interdiffusion coefficients being of the order of 10(-7) cm(2) s(-1), for the temperature range 570-620 K.