The microstructure of kerosene-based magnetite colloids, stabilized by oleic acid, has been studied using rheological, diffusion, and magnetic measurements. It is shown that more than half of all the particles are associated in quasi-spherical microaggregates with Neel’s relaxation mechanism of the magnetic moment. These aggregates, containing up to several tens of particles, are formed due to molecular forces and are stable with respect to temperature alteration. Another type of aggregate is represented by short stiff chains. They have been observed in colloids which contain at a least small number of large (similar to 20 nm) particles. These aggregates, comprising up to 4-5 particles, are formed in a reversible way under temperature variation and are characterized by a Brownian relaxation mechanism of the magnetic moment.