The concentration and wavelength dependence of the degree of polarization of light at room temperature in diluted Fe 3 O 4 particle magnetic fluids has been investigated. The volumic fraction ε of the samples investigated ranged from 0.548 to 10.36 10 - 3 . The range of wavelengths λ used in this study was 366-905 nm. Applied measuring magnetic fields up to 8 k Oe were used. For a given concentration, the degree of polarization, D, increases rapidly with the field at low fields, then less rapidly at intermediate fields, and tends to saturate at high fields. Also, D increases with increasing concentration. The results also show that the degree of polarization increases with frequency at low frequencies (in the optical region), reaching a maximum at ∼ ν = 6 10 1 4 Hz, and then decreases for higher frequencies.