A method of determining slow neutron spectra from foil activations is described. Two sets of activants are irradiated; one set is composed of nuclides each of which has only one predominant resonance in the few electron volt (eV) region (resonance set); the other is composed of nuclides which, as a group, present a variety of absorption cross sections for neutrons in the subcadmium energy range (subcadmium set). For reliability of analysis and insensitivity to experimental uncertainties, a suitable form for the neutron spectrum is found to be basically a Maxwellian plus 1/E form with variable parameters. The spectrum is written as where K and b are parameters obtained directly from the absorption rates of the cadmium-covered resonance detectors. The remaining spectral parameters G, E0, and c are chosen to produce a spectrum from which are computed absorption rates of the irradiated foils in the subcadmium set. Values of G, E0, and c are varied until the computed absorption rates give a least-squares fit to the measured absorption rates. The activation method has been applied in two reactor cores. The predicted spectra are compared with corresponding spectra calculated by SWAK and SWAKRAUM. There is reasonably good agreement between the spectra predicted by experiment and calculation.