Self-diffusion coefficients D of CD4 and ND3 were measured with the NMR-PGSE-technique over a wide range of temperature and pressure (10-200 MPa, 150-450 K for CD4, 10-200 MPa, 200-450 K for ND3). When compared to the protonated species, both substances show a dynamic isotope effect D-r = DX-H/DX-D that was found to rise to 1.3 (CD4) and 1.4 (ND3) at the lowest temperatures studied. This behavior is similar to a number of other simple liquids (HF, CH3OH, H2O). Classical theories for single particle motion in liquids suggest a dependence of D-r on the square root of the inverse mass ratio, or the square root of the inverse ratio of the moments of inertia, if translation-rotation coupling is dominant. D-r should, however, be temperature-independent. The strong temperature-dependence of D-r and its high value at low temperatures found in many liquids can thus not be explained by single particle properties, but rather has to be viewed as a collective phenomenon. It was suggested earlier that the stronger hydrogen bonds expected in the deuterated liquids are responsible for this behavior. However, the fact that methane shows a similar dynamic isotope effect is an indication that more complex mechanisms are responsible for the deviations from classical models of liquid dynamics. Quantum mechanical calculations suggest that backscattering effects may describe this interesting phenomenon.