Thermoluminescent decay of five different types of soils (soil no. 4FF, Yolo sandy loam, Hanford sandy loam, soil no. 9RDG, and Egbert muck) irradiated by Co60 photons and in the reactor were examined.Although the glow output and the glow curve shapes differed considerably among the soils, the shape of the thermoluminescent decay curves, in general, were quite similar, whether the soil was irradiated by Co60 photons or in the reactor. Regardless of the soil type, the most rapid decay occurred within the first 24 hr after irradiation. After the lapse of sufficient decay time, an apparent isothermal equilibrium state was approached. Curves were fitted to the decay data by the use of a computer program which obtains a weighted least squares fit of a function to the data by means of stepwise Gauss-Newton iterations on the parameters. The results indicated that the thermoluminescent decay curves of soils were hyperbolic [Y= α(β +X)γ+ δ] in form.