The diffusion coefficient of radioactive Sb‐124 in Si has been investigated in the temperature range 1190° to 1398 °C, with use of a technique involving measurement of the residual integrated activity of the 0.602 Mev gamma before and after removing successive 2‐ to 4‐µ layers perpendicular to the diffusion path. Results are compared to literature data obtained via p‐n junction measurements. The diffusion coefficients are on the average about 85% higher than the literature values, while the activation energies are essentially the same. The temperature dependence of the diffusion coefficient is given by with an average error of ±12%. Surface concentrations of Sb were in the range of 1019 to 1020 atoms cm−3.