AbstractNMR spin‐lattice relaxation timeT1and phase diagrams of deuterosubstituted tetrahydrofurans (THF‐hg, THF‐h4d4, THF‐d8) in D2O and D2O/salt (Salt = MeCl, Me = Na, K, Rb, Cs; CaCl2, AlCl3; MeOH, Me = Na, K) mixtures have been studied close to their lower critical solution points. It has been found that deuteration of THF leads to a shrinkage of the closed‐loop phase diagram THF‐h8‐xdx/D2O. It is an opposite and much stronger effect to the well‐known replacement of H2O by D2O in the system THF‐h8/H2O (D2O). The lower and the upper critical solution points (TC,L,TC,U) shift from 63.7 °C and 143.5 °C in THF‐h8/D2O to 84.5 °C and 128.5 °C in THF‐h4d4/D2O respectively, and they disappear in THF‐d8/D2O mixtures. This phenomena has been studied by precise (±0.01 °C) measurements of dependencies ofT1(2H) of THF and D2O deuterons on temperature. An increase of activation energies (Ea) of reorientational motion of THF‐d8has been found by introduction of ions into the solutions. The theoretical modeling of phase diagrams using renormalization‐group and decorated lattice methods shows that the observed changes ofTC,Lcan not be considered as a pure energetical effect where onlyEachanges due to additional interaction of polar molecules with the electric field of ions. The entropic contribution, viz. the appearance of additional orientations has