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Relaxation times in KH3(SeO3)2 crystals were measured as functions of temperature. There are no significant changes in T1 for 1H and 39K nuclei at TC, except for a change in the number of proton and potassium signals. The role of structural changes in this transition is relatively minor. The transition is driven by the protons in the O(2)–H(2)–O(2) bonds, and the change in the number of resonance lines results from the crystal's ferroelastic properties. The results show that the ferroelastic phase transition is associated with the ordering of the O(2)–H(2)–O(2) hydrogens. The T1 and T2 of 1H in the O(2)–H(2)–O(2) bonds are liquid-like, i.e. of the order of milliseconds. In addition, 39K nuclear magnetic resonance results confirm that the phase of this crystal below TC is ferroelastic and that the phase above TC is paraelastic. These results are compared with those obtained for deuterated KD3(SeO3)2 crystals.