A technique is presented for the evaluation by activation methods of the thermal resonance reactions in a system with thermal resonance absorbers; the method consists of measuring twice the ratios between the activity of a detector irradiated bare and that of a detector covered by a proper filter. These measurements can be useful in the analysis of spectral effects in plutonium fueled lattices. For instance, using a gadolinium filter and a cadmium filter the two resulting cutoff energies (0.2 and 0.6 eV, respectively) can be obtained such as to enclose the resonance of 239Pu at 0.3 eV. A calculational model to be used in connection with available cross section libraries is described, and a detailed investigation is presented of the behavior of gadolinium filters for detectors having resonances in the thermal energy range. Preliminary applications of the calculational model are presented whence indications are obtained concerning the choices of the detectors and of the filter thicknesses. The feasibility of the technique is shown from the experimental standpoint and the technique is applied by measurements in a neutron system where the spectrum is well known. The reliability of the calculational model adopted is also shown, since for different detectors and for different filter thicknesses the experimental values agree well with the corresponding calculated results.