Equations are derived for the surface activity of a diffusion specimen for two cases: In the thin plate case (plane source at the origin of a semi‐infinite slab), an analytical solution is obtained for a statically annealed sample and for a sample annealed during simultaneous deformation (either compression or tension normal to the free surface). In the thick plate case, solutions for similar situations are obtained (under proper experimental conditions) in the form of a definite integral convenient for evaluation on an automatic digital computer. Specific cases are shown (using nickel) to illustrate the range in which the thin‐ and thick‐plate solutions are valid. When surface intensity methods are used with a plate sufficiently thick that considerable absorption takes place before diffusion, it is necessary to know precisely the initial plate thickness. This is true regardless of the diffusion time; this is not necessary when the sectioning method is used (with sufficiently long diffusion times). A discussion of a surface intensity experiment by Wazzan is given to illustrate this point.