The complex magnetic susceptibility of two magnetic fluids, with different degrees of colloidal stabilization, has been measured over the frequency range 100 MHz to 6 GHz. The colloidal stabilization of the magnetic fluids has been investigated using magneto-optical measurements. Based on complex magnetic susceptibility measurements, χ(ω)χ′(ω)−iχ′′(ω), the dependence of the maximum absorption frequency at resonance, f_max, and of line width, Δf, on an external magnetic polarizing field, H, over the range 0–1.45 kOe, has been examined for both magnetic fluids. The experimental results have been interpreted in terms of magnetic interparticle interactions and particle agglomeration.