X-irradiation of tetramethylammonium hydroxide pentahydrate (TMNOH) at 77 K produces trapped electrons, and CH3. and (CH3)(3)N+CH2. radicals. The trapped electrons were detected by bleaching with visible light and by subtraction of spectra from bleached and unbleached samples. The decay of the methyl radicals starts at approximate to 100 K and that of the (CH3)(3)N+CH2. radicals at approximate to 150 K. The ESR spectrum measured at 130 K has the best resolution and is a superposition of contributions from the methyl radicals (13%) and from the (CH3)(3)N+CH2. radical (87%). The reversibility of the line shapes with temperature variations suggested that the increased resolution observed at 130 K is due to dynamical effects involving the (CH3)(3)N+CH2. radicals. ESR spectra from these radicals can be simulated by assuming that the hyperfine tensor components for the two a protons are averaged by two types of motions : rotation of the CH2 group about the C-2v symmetry axis and precession or wobbling of this axis. The parameters used in the simulation are g(iso) = 2.0022, two equivalent a protons with axial hyperfine components A(parallel to)(av) = 25.0 G and A(perpendicular to)(av) = 23.0 G, one N-14 nucleus with A(N) 3.9 G, and one remote proton with A(H,remote) = 4.2 G. The remote proton is identified with a lattice proton. If the precession model is adopted, the precession angle calculated from the principal values of the hyperfine tensor for the ct protons used in the simulations is 47 degrees. For the wobbling model, the wobbling angle deduced is 72 degrees. These results suggest that the hydrogen-bonded cage around the guest allows large-scale dynamical effects even at 130 K.