Proton transfer in the solid HI-enriched rhodium iodide (H0.81RhI3.81) at 150-400 K was studied using conductivity measurements and H-1 NMR. The compound displays rather high proton conductivity (10(-4) S/cm at 360 K) associated with the activation energy for proton migration of approximate to 0.32 eV which is very close to the activation energy reported earlier for solid iodine doped with HI. From quantum-chemical calculations it was supposed that the interaction between HI molecules and I- ions results in symmetrical or quasi-symmetrical H-bonded complexes [HI2](-), and the conductivity mechanism is apparently a two-stage proton transfer: proton displacement along the hydrogen bond followed by HI reorientation. (c) 2006 Elsevier B.V. All rights reserved.