The microscopic mechanisms of proton transport in partially deuterated ammonium hydrogen selenate [NH 4 HSeO 4 (AHSe)] and partially deuterated mixed crystals of betaine phosphate (DBP) and betaine phosphite (DBPI) (DBP 1-x DBPI x ) were studied by means of one-dimensional Fourier transform 2 H NMR, two-dimensional (2D) 2 H NMR and dielectric measurements. Slow chemical exchange processes of deuterons between different hydrogen bridges occur which can be studied by means of the 2D 2 H NMR technique over a wide temperature range. In both systems, AHSe and DBP 1-x DBPI x , the ionic conductivity is probably realized via jumps of the protons or deuterons between different hydrogen bonds (inter-bond jumps). The importance of certain defects for the conductivity process is clearly revealed. In AHSe, jumps occur through non-occupied hydrogen positions. In the mixed crystals DBP 1-x DBPI x , the creation of defects is in fact favoured by the substitution of DBP for DBPI molecules or vice versa. The creation of vacancies may be explained by the occurrence of defects in the lattice structure, e.g. on the basis of the known Grotthuis mechanism and a multi-dimensional conductivity model as suggested for KDP.