A detailed analysis of the structure and speciation of La3+ clusters in the 0.1 mol L–1 La(NO3)3 salt methanol (MeOH) solution has been performed by means of molecular dynamics (MD) simulations. The time distribution and NO3–/MeOH ligand composition of these clusters have been computed using graph theory techniques. These analyses revealed the formation of branched-like polynuclear clusters in the solution, the predominant clusters being the 3, 7, and 8 La3+ clusters. In these clusters, the La3+ cations are bound by “monodentate” nitrate bridges. Moreover, the mechanism of aggregation of the La3+ clusters has been examined with the development of a 3-step model. Finally, the origin of the aggregation process has been identified by estimating the binding constant for the ion pair La3+-NO3– using the Bjerrum theory of dilute solutions, with pK° = 5.32 at 25 °C. The low value of the dielectric constant of methanol promotes the binding of the ion pair La3+-NO3– and the nitrato-bridging polymerization, resulting in the formation of clusters.