The liquid liquid phase diagrams of 1-hexyl-3-methylimidazolium bis(trifluoromethyl)sulfonyl imide with regular and deuterated 1-alkanols (CnH2n+1OH and CnH2n+1OD, n = 4-10) have been obtained. Deuteration of alcohols in the hydroxyl groups changes the upper critical solution temperatures in a very surprising way. The immiscibility region of the systems upon deuterium substitution in alcohol initially decreases (n = 4, 5), then significantly enlarges (n = 7), and finally practically does not change (n = 8-10) in comparison with the systems with nondeuterated alcohols. This is the first time when non-monotonic change of the isotope effect on miscibility with the increase of the length of the alkyl chain was observed. (C) 2014 Elsevier B.V. All rights reserved.