The frequency-dependence of the mean dipole-polarizability oc(a) of the organometallic compounds Ln(C5H5)(3) (Ln = Nd, Sm, and Er) is studied by means of refractive index measurements in the vapour phase. From our white-light interferometric experiments the electronic static dipole-polarizabilities are obtained as alpha (0) = 367.7(2.8), 356.2(1.4), and 316.4(5.0) (in 10(-41) C-2 m(2) J(-1)) in the sequence Nd, Sm, and Er. This result reflects that an increase in the number of f-electrons in the series of the Ln(3+) ions leads to a decrease in alpha (0). This observation is in good accordance with polarizabilities deduced from refractive indices and dielectric constants of the corresponding oxides and fluorides. However, it is opposite to what is found from studies of alpha(sigma) of these three compounds in tetrahydrofuran solution.