Magnetic particle-effector conjugates are widely used in vitro for cell sorting in various pathologies. The coupling between the particles and the effectors being realized through S-S bridges, the particles must first be thiolated before the coupling. In this work, the synthesis, in aqueous medium, of nanoparticles of maghemite thiolated by dimercaptosuccinic acid is described. The superficial complexation by a thiol-containing ligand induces a reductive dissolution of the oxide and leads to the adsorption of polydisulfide species coming from the oxidation of the ligand. Adsorption and redox reactions being strongly correlated to the composition of the medium, the amount of adsorbed ligand and the quantity of iron(II) released into the medium have been simultaneously determined, at various pH, for different concentrations of ligand added. The charge of the particles is drastically modified in the presence of a chelating agent; as a consequence, the colloidal stability is greatly affected and so the flocculation ranges of the complexed particles have been established for different pH. When the quantity of ligand added is sufficient (0.05 mol/mol of iron), the ferrofluid based on thiolated maghemite particles is stable between pH 3 and 11 and can be used for biomedical applications. Copyright 1997 Academic Press. Copyright 1997Academic Press