A measurement of the Rb-87 spin-lattice relaxation rate in a Rb-0.58(N
D4)(0.42)D2PO4 single crystal in a wide temperature range allowed for
a discrimination between classical thermally activated deuteron jumpin
g across the potential barrier between the two equilibrium sites in th
e H-bond and phonon assisted incoherent deuteron tunnelling. The resul
ts show that thermally activated deuteron intra-bond jumping dominates
above 55 K whereas at lower temperatures phonon assisted deuteron tun
nelling becomes rate determining.