In this article we report the results of two size selection methods that are based on interfacial interaction between nanosize particles, magnetite in this case, anionic surfactants, and nonpolar solvents. It is proposed that by selecting a suitable surfactant type and/or conditions to modify the particle-particle separation distance, only smaller particles can be stabilized against aggregation and settling making a size sensitive separation possible. Using this phenomenon, an effective size selection at the nanosize level has been achieved and the preliminary results are presented here. Depending on the conditions, stable suspensions of nearly monodispersed nanoparticles of magnetite (diameter less than 10 nm and standard deviation, σ, below 0.2) were obtained from polydispersed powders (less than 40 nm in diameter and σ around 0.6) synthesized from aqueous solutions at 25°C. Magnetization measurements of the fractions confirmed the effectiveness of the developed size selection methods. © 2002 American Institute of Physics.