We report on a comparative study of the narrow-band semimetals FeSb2 and its structural homologue RuSb2 by means of 121,123Sb nuclear quadrupole (NQR) and nuclear magnetic resonance (NMR) spectroscopy. From NQR for both compounds two temperature regimes could be identified by use of 123(1/T 1) measurements. Above 40 K a conventional activated behavior (with Δ/k B ≅ 400 K for FeSb2) dominates in 123(1/T 1), whereas below 40 K in both systems an unconventional 123(1/T 1) behavior with a smooth maximum at around 10 K is observed. To analyze this behavior, we propose the presence of T-dependent in-gap states forming a narrow energy level of localized spins with S = ½ near the bottom of the conduction band. These states might have originated from an inherent Sb-deficiency in both compounds. This model enables us to fit the 123(1/T 1) data in the entire investigated temperature range (2–200 K) for FeSb2. Ab initio band structure calculations reveal more than a factor of two larger Δ value for RuSb2 as compared with FeSb2. This results in dissimilar behavior of 123(1/T 1) in FeSb2 and RuSb2 above 40 K evidencing the inefficiency of thermal activation of electrons over the large energy gap at T ≤ 300 K in RuSb2 and dominating of quadrupole relaxation channel in RuSb2 in this temperature range caused by phonon relaxation involving two-phonon (Raman) scattering. In addition, extra wide range field-sweep NMR measurements are performed at various temperatures on FeSb2 and RuSb2. The complex broad spectra could be modeled and from the shift of the 121Sb central transition the 3d component of the shift K 3d (T) could be extracted