Proton conductivity and diffusion in molten nominally dry phosphonic acid (H3PO3) have been studied by H-1 PFG NMR (pulsed magnetic field gradient NMR) and ac impedance spectroscopy. The high intrinsic proton conductivity is found to be the result of fast structure diffusion of protonic defects (Grotthuss mechanism) which are present at high concentration (high degree of self-dissociation). The proton transport behavior is reminiscent to this of phosphoric acid, but slightly higher correlations in the diffusion of protonic defects, as indicated by a higher Haven ratio (similar to 2.5 compared to similar to 1.5), and the lower rate of structure diffusion (Grotthuss mechanism) compared to vehicle diffusion (similar to 90% for phosphonic acid compared to similar to 98% for phoshoric acid) are suggested to be the consequence of a "weaker" hydrogen bond network formed by phosphonic acid compared to phosphoric acid. But contrary to phosphoric acid, phosphonic acid may be immobilized to organic structures by stable P-C bonds. This opens a way to a new class of fully polymeric proton conducting materials, which are of paramount interest for PEM-fuel cell technology. (C) 2008 Elsevier B.V. All rights reserved.