Abstract: The effect of the presence of an aqueous complex near an aluminum cation ([Al(H2O)6]3+) on the microstructure and molecular mobility in a mixture of ethylammonium nitrate (EAN) and aluminum nitrate has been studied. Molecular dynamics simulation, which allows the system’s evolution to be described at the molecular level, was chosen as the research method. The results of simulations of three systems: EAN, EAN + Al(NO3)3, and EAN + [Al(H2O)6]3+[] are analyzed. To analyze in detail the changes in the microstructure after addition of anhydrous aluminum nitrate to EAN and addition of aqua complexes around aluminum ions, the radial distribution functions were calculated. In addition, several kinetic characteristics were calculated for the EAN + [Al(H2O)6]3+[] system: self-diffusion coefficients of the mixture components and rotational reorientation times of the nitrate anion. It was demonstrated that the structure and properties of the system are considerably affected by water that appears during the preparation of mixtures with aluminum salts and cannot be completely removed by standard methods in some cases. This effect should be taken into account when developing mixtures for various applications.