Reversing-pulse electric birefringence (RPEB) of a nearly monodisperse iron(III) hydroxide oxide sample in the beta -form (beta -FeOOH) was measured at 25 degreesC and at a wavelength of 633 nn in aqueous media in the presence of NaCl, The concentrations of beta -FeOOH and added NaCl varied between 0.00111 and 0.0555 gn and 0.03 and 2.0 mM, respectively. Except for the suspensions with high salt concentrations, each RPEB signal showed a dip or minimum in the reverse process upon electric field reversal, together with a smooth rise in the buildup and a fall in the decay process. The observed signals were analyzed with a new RPEB theory, which takes into account not only the permanent electric dipole moment (mu) but also the root-mean-square ionic dipole moment ((m(2))(1/2)) due to the ion fluctuation in ion atmosphere, in addition to the field-induced electronic (covalent) dipole moment Delta alpha 'E. The results showed that the slowly fluctuating moment of (m(2))(1/2) is by far the most predominant one for the field orientation of the beta -FeOOH particle, though the permanent dipole moment mu may not be completely excluded. The rotational relaxation time of the whole particle was evaluated from the decay signal, while the relaxation time for fluctuating ions was estimated from RPEB signal fitting. The sign of the steady-state birefringence for beta -FeOOH suspensions was positive without exception under the present conditions. The birefringence signals in the steady state (Sld) were proportional to the second power of the applied field strength (E) in the low field region; thus, the Kerr law was verified to hold for beta -FeOOH suspensions. The specific Kerr constant was evaluated for each suspension by extrapolating the values of delta /d to zero field (E -> O).