Four different paramagnetic centers, induced in bayerite samples by irradiation, have been characterized by EPR and ENDOR spectroscopies. Two of them, respectively, of V- and F-types, have been found to be mobile at room temperature and frozen at lower temperature. The first one, with g1= 2.021±0.002, g2=2.010±0.005, and g3=2.005±0.002, is identified with an O− species stemming from the ionization of OH− groups present in the structural cavity walls of the bayerite. The second one, with g∥=1.9970±0.0005 and g⊥=2.0020±0.0005, corresponds to solvated electrons. A third center, with g∥=2.035±0.002 and g⊥=2.0025±0.0005, with strongly hindered motion even at room temperature, is assigned to an O−2 species located at the center of the structural cavity, U. The last center is a doublet with a 502.5±0.5 Oe hyperfine splitting and giso= 2.0022±0.0001; it is due to H atoms located in U. Particular attention has been paid to the ’’matrix’’ ENDOR line at the free proton nuclear frequency and to the nature of the nuclear and electronic relaxation mechanisms that govern its shape and intensity. ENDOR spectra were simulated considering various possibilities for the proton distribution and the relaxation mechanisms. The best agreement is obtained with: (i) a nuclear relaxation time that is angular independent, (ii) a discrete lattice proton distribution in agreement with literature data; (iii) a lower limit for the distance between the paramagnetic center and the different lattice protons that results in the matrix ENDOR line (R≳3.4 Å=0.34 nm). This lower limit excludes all nearby protons that experience direct contact hyperfine interaction; (iv) an upper limit for the distance R. It corresponds to the most distant protons that are involved in the ENDOR mechanism via electron–nuclear dipolar interaction. This limit is shown to increase when microwave power is increased; (v) spin–packet linewidths that are shown to be dependent on the electron spin–lattice relaxation time.