The collective diffusion coefficient D(C) of diluted suspensions of positively charged iron oxide maghemite particles was experimentally investigated using a capillary electrophoresis instrument on the grounds of Taylor dispersion theory. Conditions for this approach to be applicable to nanoparticles of mean solid diameter below 10nm were set in this work, enabling precisions on D(C) determination of less than 2% relative standard deviation (RSD). Significantly different D(C) values were thus measured for particle populations differing in solid number mean diameter by only 2 nm. The obtained values were compared to the z-average diffusion coefficient derived from dynamic light scattering (DLS) experiments and used for the calculation of the Stokes radius. The measured diffusion coefficients appeared to be dependent on particle volume fraction and electrolyte ionic strength. These observations were eventually discussed in terms of particle interactions.