The effect of the addition of cosolvents in the room temperature ionic liquid (RTIL) 1-ethyl-3methylimidazolium ethyl Sulfate ([Emim][EtSO4]) was probed by the solvent and rotational relaxation studies of coumarin 153 in neat ionic liquid [Emim][EtSO4] and [Emim][EtSO4]-cosol vent mixtures by using steady-state and time-resolved fluorescence spectroscopy. With gradual addition of cosolvents in the RTIL, both the average solvation time and rotational relaxation times gradually decrease. Addition of cosolvents in the IL decreases the viscosity of the medium. We have optimized the geometry of [Emim][EtSO4] and [Emim][EtSO4]-cosolvent mixtures by using quantum chemical calculations using density functional theory methods, which show the formation of hydrogen bond between cosolvents with [Emim][EtSO4]. With addition of the same amount of alcohols in neat [Emim][EtSO4], the rotational relaxation time decreases more compared to the addition of the same amount of water.