The diffusion of different deuterium-labeled (OD) low-molecular-weight alcohols and D2O in Nylon 11 has been investigated by monitoring the NH/ND exchange in the polymer by Fourier transform near-infrared (FTNIR) spectroscopy. The results demonstrate that the diffusion process of the different penetrants is strongly controlled by their molecular structure and geometry. To characterize this phenomenon quantitatively, the diffusion coefficients for the different deuteration agents have been determined for Nylon 11. The rate of the diffusion was found to decrease with increasing size of alcohol. Furthermore, it could be shown that with this technique the less ordered regions of the polymer can be separated spectroscopically from the crystalline domains, which are not accessible for the isotopically labeled diffusants.