The dynamic isotope effects (IE) on the translational and rotational motion in liquid ammonia and water are reassessed by NMR measurements. For H2O/D2O the translational and rotational IE are clearly distinct. At 298 K, 23% and 30% are obtained, respectively. Both effects as well as the slopes of the temperature dependencies increase with decreasing temperature. For NH3/ND3 a rotational IE of 37% was observed at 298 K. A small increase to 40% at 222 K could be ascertained. The translational IE is about 15% at room temperature and exhibits a stronger temperature dependence. It is suggested that the observed deviations of the IE's from the square root of mass and square root of moments of inertia laws are caused by translation-rotation coupling as well as quantum effects. The experimental data obtained in the present paper are also of importance for the correct interpretation of all kinds of experiments on water and ammonia, where isotopic substitutions are involved. (C) 2001 American Institute of Physics.