Preparation and properties are discussed of a novel magnetic dispersion, containing surface-modified silica colloids with a core of single-domain magnetite particles. The underlying idea is to tailor the silica shell thickness and surface properties such that the colloids are stable spheres with isotropic interactions, whereas an external magnetic field produces weak dipolar attractions and consequently reversible anisotropic structures. The influence of the shell thickness is analyzed in terms of a dipolar-sphere potential. Preparation of magnetite cores, silica growth, and surface modifications with an alcohol and a silane are described in detail. Particle properties are investigated, among other things, with SAXS, light scattering, and electron microscopy. The major conclusion is that the preparation route yields stable, nonaggregated magnetic silica particles with a shape and internal structure which is mainly determined by small magnetite clusters in the starting ferrofluid.