A new method to estimate undisturbed formation temperatures from measurements taken during the return to equilibrium is presented. The proposed conceptual model considers, as an approximation, the return to equilibrium under spherical-radial heat flow with unit initial normalized temperature in the perturbed region and zero outside. Analysis shows that temperature transients travel linearly as 1/square-root t at large times and the undisturbed formation temperature can be obtained directly from the intercept of a T vs 1/square-root t plot. Circulation time is not explicitly required in the approach presented in this paper, which represents an advantage since, in practice, its estimation is uncertain and difficult to evaluate with sufficient accuracy using conventional methods. Data from examples previously reported in the literature are analyzed to illustrate the method.