Non-photochemical spectral hole-burning is reported for the R-1-line of [Cr(oxalate)(3)](3-) in frozen 2:1 (v/v) ethylene glycol/H2O, ethylene glycol/D2O ethylene glycol-d(2)/D2O and ethylene glycol-d(6)/D2O at 3 K. The hole-burning efficiency decreases upon deuteration of the host molecules, indicating that tunnelling of the proton/deuteron in the double well potential of hydrogen bonds between the oxygen atoms of the ligands and the surrounding solvent molecules is the responsible mechanism. The perprotonated and perdeuterated sample show hole-burning efficiencies of ca. 0.2% and 0.018%, respectively. The present results corroborate the assignment of a thousand-fold increase in the hole-burning efficiency upon partial deuteration of NaMgAl(oxalate)(3) center dot 9H(2)O:Cr(III) to 180 degrees water flips. (c) 2005 Elsevier B.V. All rights reserved.