Electron paramagnetic resonance is used to investigate ionic cobalt ferrite-based magnetic fluids. Two magnetic fluid samples having particles with different average radius of 6.8 nm and 7.8 nm were doped with Cu2+ at different concentrations. Changes on the resonance line shape of the Cu2+-ion in the aqueous phase was analyzed taking into account the particle polydispersity, the contribution of the dipolar held due to the surrounding magnetic nanoparticles, and the electrostatic interaction between the surface charged magnetic nanoparticles and the Cu2+-probe. The particle polydispersity parameters we found from the resonance line shape analysis are in excellent agreement with the parameters obtained from electron microscopy. The Cu2+ spatial distribution function around the magnetic nanoparticles gives very reasonable values for both, the particle-particle distance and the ionic layer thickness at the particle surface. (C) 1997 American Institute of Physics.