Proton NMR relaxation times ( T2, T1, T1 ϱ) are reported for powder samples of MoO 3 · 2H 2O and yellow MoO 3 · H 2O in the temperature range 150-325 K and at 20 and 60 MHz. No translation of hydrogen atoms is detected but the spin-lattice relaxation behavior indicates reorientation of H 2O molecules. The waters coordinated to Mo atoms undergo 180° flips (about their C2 axes) with similar motional parameters in both compounds. The interlayer waters in MoO 3 · 2H 2O undergo 180° flips with different parameters. An assumed Arrhenius-type temperature dependence of correlation times leads to preexponential factors which are "anomalously" low. The possible involvement of temperature-dependent activation barriers is discussed.