Dielectric spectra over the frequency range of 0.2 less than or similar to v/GHz <= 89 have been Measured for the room-temperature ionic liquid 1-N-ethyl-3-N-methylimidazolium ethylsulfate ([emim][EtSO4], IL) and its mixtures with dichloromethane (DCM) at temperatures of 5 less than or similar to v/degrees C <= 65 and 25 degrees C respectively. The spectra of the neat IL at all temperatures and those of the mixtures could be satisfactorily fitted by assuming three relaxation modes, a Cole-Cole process at lower frequencies and two Debye processes at higher frequencies. Consistent with previous studies, detailed analysis of the first (lowest-frequency) process, centered at 0.2-2 GHz depending on temperature and composition, indicated that it is mainly due to the reorientation of the dipolar [emim](+) cations. At high dilutions in the mixtures (x(IL) less than or similar to 0.2), contact ion pairs also contribute to this mode. The second mode at similar to 8 GHz, which is absent from the dielectric spectra of previously studied imidazolium salts and their mixtures with DCM, is assigned to reorientation of the dipolar [EtSO4](-) anions. The highest-frequency mode (located at similar to 80 GHz) in the mixtures is a composite of low-energy intermolecular vibrations originating from the IL and the rotational diffusion of DCM molecules. Detailed analysis of the spectra reveals marked, preference for parallel orientational correlations of the IL components, with the cation dipoles showing a strong and the anions showing preference for antiparallel arrangements. These effects are the probable cause of the unusually high dielectric constant of [emim][EtSO4]. The Structure of the IL appears to be maintained up to quite high dilutions (x(IL) >= 0.2) in DCM.