An experimental method for the measurement of thermal-neutron spectra by neutron-induced radioactivities in selected isotopes is described. Criteria from theoretical and experimental considerations are developed and applied for the selection of such detectors. The applications and limitations of the method are discussed. The experimental technique has been applied to investigate neutron spectra in borated H2O solutions, using four detectors: 164Dy, 176Lu, 151Eu, and 197Au. The spectra obtained are in good agreement with the known spectra.