The perturbation in a thermal neutron spectrum near an absorber immersed in a moderator has been studied using activation techniques. The results show two separate effects for pure thermal spectra, a ‘transmission hardening’ depending on the energy variation of the absorber cross section and a ‘boundary effect’ depending on the energy variation of the moderator scattering cross section. These results are in agreement with energy-dependent Milne problem calculations for a mass one, free-gas moderator and with another experimental result when a third ‘source hardening’ effect in reactor spectra is considered.