Control over nanoparticle size is a key factor which labels a given preparation technique successful. When organic reactions are mediated by ultradispersed catalysts, the concentration of the colloidal nanoparticle catalysts and their stability become key factors as well. In this study, variables affecting iron hydroxide nanoparticle size, stability, and maximum possible colloidal concentration in AOT/water/isooctane microemulsions were investigated. Iron hydroxide was prepared in single microemulsions by first solubilizing iron chloride powder in the water pools, followed by addition of aqueous NaOH. Upon addition of NaOH, Fe(OH)3 nanoparticles stabilized in the water pools formed in addition to bulk precipitate of Fe(OH)3. The time-invariant concentration of the stabilized Fe(OH)3 is defined as the nanoparticle uptake, and it corresponds to the maximum possible concentration of the colloidal nanoparticles. The effect of the following variables on the nanoparticle uptake and size distribution was investigated:  mixing time; surfactant concentration; water to surfactant mole ratio; and the initial concentration of the precursor salt. At 300 rpm of mixing a constant uptake of iron hydroxide nanoparticles was achieved in about 2 h and further mixing had limited effect on the nanoparticle uptake and particle size. An optimum R was found for which a maximum nanoparticle uptake was obtained. Nanoparticle uptake increased linearly with the surfactant concentration and displayed a power function with the initial concentrations of the precursor salt. The surface area/g of the nanoparticles was much higher than literature values, however, following a trend opposite to that of the nanoparticle uptake. The surface area/unit volume of the microemulsion, on the other hand, followed the same trend as the nanoparticle uptake. The particle size increased as R and/or the surfactant concentration increased. A mathematical model based on correlations for water uptake by Winsor type II microemulsions accurately accounted for the effect of the aforementioned variables on the nanoparticle uptake.