It is demonstrated how a quantitative study of some anomalous thin‐foil annealing phenomena, if interpreted correctly, leads to the determination of important physical constants. The necessary theory is developed and applied to measure the activation energy for dislocation pipe diffusion in thin aluminum foils from the annealing rate of quenched‐in and subsequently truncated faulted dislocation loops. The results are compatible with those obtained from void annealing by another group. Triangular faulted defects present in bulk‐aged and thinned samples of aluminum are shown to consist of a mixture of extrinsic and intrinsic loops. They can readily be distinguished by their annealing rates. From the relative values of these rates, the relation between the intrinsic and extrinsic stacking‐fault energies has been determined.