Abstract The nuclear magnetic relaxation rates of the 139La ion in aqueous solutions of halides, nitrate, sulfate, and perchlorate have been measured. The observed relaxation rates increased as the concentration of the salts increased. However, in the low concentration range, the relaxation rate divided by the relative viscosity, Δv/ηr increased only in the cases of nitrate and sulfate, while it decreased in the cases of halides and perchlorate. It was found that the formation of inner-sphere complexes is a satisfactory explanation of the increase in the Δv/ηr of nitrate and sulfate solutions in the low to middle concentration range. With the fact, proved by various authors, that the lanthanide ions have several coordination numbers, the decrease in the Δv/ηr of halide and perchlorate solutions in the same concentration range can be explained in terms of the existence of an equilibrium between two hydrated species, such as: La(H2O)93+\ightleftharpoonsLa(H2O)83++H2O and the symmetrical species, La(H2O)83+, contributes to the decrease in Δv/ηr. At a high nitrate concentration, after reaching the maximum value Δv/ηr decreases, demonstrating that a symmetrical trinitrato species may be formed in the solution.