The diffusion of different deuterium-labeled (OD) low-mol.-wt. alcs. and D2O in polyamide 11 (PA11) has been investigated by monitoring the NH/ND-exchange in the polymer by Fourier-Transform near-IR (FTNIR) spectroscopy. The results demonstrate that the diffusion process of the different penetrants is strongly controlled by their mol. structure and geometry. To characterize this phenomenon quant., the diffusion coeffs. for the different deuteration agents were detd. for PA11. Furthermore, it could be shown that with this technique the less ordered regions of the polymer can be sepd. spectroscopically from the cryst. domains which are not accessible for the isotopically labeled diffusants.