The adsorption of N-oleoylsarcosine onto in water dispersed magnetite nanoparticles were investigated at different concentrations and pH values. The coated nanoparticles surface became hydrophobic at a molar ratio of N-oleoylsarcosine:Fe3O4 below 0.22:1 within a pH range of 5-9.5. Aqueous-based magnetic fluids could not be obtained. A hydrophilic surface was observed at a molar ratio of N-oleoylsarcosine: Fe3O4 above 0.37:1 and at pH levels ranging from 6 to 8.5. Stable aqueous-based magnetic fluids could be prepared. The magnetite particles flocculate at pH levels below 5.5 or above 8.5. The hydrophobic behavior was explained assuming a monolayer adsorption mode. On the other hand, the hydrophilic properties were caused by a bilayer adsorption structure. This hypothesis was in overestimation with results of infrared (IR) spectroscopy and photon correlation spectroscopy of the prepared magnetic fluids. The influences of pH levels on the formation of N-olcoylsarcosine micelles were also discussed to explain the pH-dependant formation of bilayer adsorption structure, i.e. surface small micelle structure. It was indicated that it became difficult for N-oleoylsarcosine adsorb at the water-air interface, or adsorb at the nanoparticles surfaces in the form of surface small micelles at pH levels above 8.5. Besides, the aqueous-based magnetic fluids were also characterized by magnetization curve determinations. (C) 2003 Elsevier B.V. All rights reserved.