High-resolution quasi-elastic neutron scattering (QENS) will be used to study the dynamics in pure RTILs [Bmim][PF6] and [Bmim][TfO] and their solutions in acetonitrile-d3 (AN). The aim is to elucidate the rotational and translational diffusive motion of 1-butyl-3-methyl-imidazolium cation [Bmim]+ as a function of composition and temperature. The dynamic information in conjunction with the NMR results and molecular dynamics (MD) computer simulations will make it possible to analyse in more detail the influence of solvation and association phenomena on the practically important properties of the investigated mixtures of RTILs with AN. It is imperative to obtain reliable experimental dynamic data in order to compare with the simulation results and to refine the Force Field Models used in MD simulations to understand a microscopic basis for the binary mixtures of RTILs with molecular liquid