The H/D exchange process in the imidazolium-based room temperature ionic liquids (RTILs) 1-decyl-3-methyl-imidazolium bromide- and chloride ([C(10)mim][Br] and [C(10)mim][Cl]) in D2O solutions of various concentrations was studied applying H-1, C-13 NMR, and Raman spectroscopy. The time dependencies of integral intensities in NMR spectra indicate that the H/D exchange in [C(10)mim][Br] at very high dilution (10(-4) mole fraction of RTIL) runs only slightly faster than in [C(10)mim][Cl]. The kinetics of this process drastically changes above critical aggregation concentration (CAC). The time required to reach the apparent reaction saturation regime in the solutions of 0.01 mole fraction of RTIL was less 10 h for [C(10)mim][Br], whereas no such features were seen for [C(10)mim][Cl] even tens of days after the sample was prepared. The H/D exchange was not observed in the liquid crystalline gel mesophase. The role of anions, self-aggregation (micellization), and mesophase formation has been discussed. Crucial influence of Br- and Cl- anions on the H/D exchange rates above CAC could be related to the short-range ordering and molecular microdynamics, in particular that of water molecules. The concept of the conformational changes coupled with the H/D exchange in imidazolium-based ionic liquids with longer hydrocarbon chains can be rejected in the light of C-13 NMR experiment. The revealed changes in C-13 NMR spectra are caused by the secondary (C-13) isotope effects not being the signal shifts due to the conformation trans-gauche transition.