[en] Mean deactivation rate of rare-earth element (REE) ions in their excited states, which is single-valued function of their complexation, is employed as an indicator of the complexation in the respective solutions. The first four stability constants of the aquacomplexes Eu³⁺ in acetone at 293 deg K were found by means of such procedure: β₁ = (1.27+-0.2) x 10³, β₂ = (5.6+-0.65) x 10⁴, β₃ = (4.41+-0.75) x 10⁵, β₄ = (5.92+-0.86)x 10⁵. Upon increase of water concentration in europium nitrate acetone solution, at first three water molecules are inserted into the REE ion coordination sphere, then NO₃^3- anions are replaced by water. This is confirmed by the rate of nonradiative energy transfer between Eu³⁺ and Nd³⁺ in acetone depending on water concentration