Experimental observation of the 159 Tb(n, 2 n) reaction product was performed with application of the activation technique. Tb specimen of natural composition was irradiated with (d, d) neutrons of 5.39 and 7 MeV energies. Instrumental spectra of Tb specimen were measured with HPGe spectrometer. An unexpected 944.2 keV γ-ray peak was observed. Other γ-ray lines due to 158g Tb decay were identified as well. A bonded dineutron emission with the binding energy (B dn ) within limitations 1.3 MeV < B dn < 2.8 MeV is evidenced by the energy of incident neutrons and by the 158g Tb presence in the output channel. The specific nuclear properties of 158 Tb as deformed nucleus were discussed to explain a bonded dineutron formation based on theoretical assumptions and calculations, using standard parameters for this mass region.