The influence of the post-synthesis adsorption of Co(ii) ions on the structural and magnetic properties of maghemite (γ-Fe2O 3) nanoparticles with a mean particle size of about 10 nm has been investigated. It is shown that the step-wise adsorption of Co(ii) can controllably increase the blocking temperature, TB, of the system up to 60 K with respect to that of untreated particles, while neither the particle size nor the particle size distribution are significantly modified. This is accompanied by a four-fold increase in the coercivity, HC, at low temperatures. Using a selective leaching of the previously adsorbed Co(ii) ions the TB and HC values of the pristine γ-Fe 2O3 nanoparticles are recovered. Hence, a reversible and controllable tailoring of the magnetic properties (e.g., TB and HC) of the γ-Fe2O3 nanoparticles can be achieved by a simple adsorption and desorption process of Co(ii) ions after their synthesis. © The Royal Society of Chemistry.