An ionic ferrofluid is prepared using coprecipitation and acid treatment techniques. In the ferrofluid, Co3+ and Fe2+ dissolved from CoFe2O4 nanoparticles via suitable acid attack, are absorbed on the surface of the magnetic particles and thus prevent them from aggregating. We call this self-formed ferrofluid. The colloidal particles in the ferrofluid are constructed with a magnetic core and a nonmagnetic surface layer about 2 nm. The size and the volume fraction of particles in the ferrofluid are described with geometric diameter d and volume fraction ϕv, as well as magnetic diameter dm and magnetic volume fraction ϕm. The viscous and magnetic properties of this ferrofluid depend on these parameters. The viscosity can be tunable for any volume fraction of particles by adding poly(ethylene glycol) into the carrier liquid.