Control over nanopa1ticle size 1s a key factor which labels a given nanoparticle preparation technique successful. When organic reactions are mediated by ultradispersed catalysts the concentration of the colloidal nanocatalysts and their stability become key factors as well. Ultradispersed metal oxide nanoparticles have applications as heterogeneous catalysts for organic reactions, and were recently demonstrated as effective H₂S_(g) absorbents. The catalytic activity and absorption effectiveness of metal oxide nanoparticles depend primarily on their surface area, which in turn, is dictated by their size, colloidal concentration and stability. This work presents a water-in-oil (w/o) microemulsion approach for in-situ preparation of ultradispersed metal oxide/hydroxide nanoparticles, namely:​ iron and copper and discusses the effect of different (w/o) microemulsion variables on their stability and highest possible time-invariant colloidal concentration (nanoparticle uptake). The concentration of the stabilized metal oxides corresponded to the nanoparticle uptake. In-situ preparation of colloidal catalysts and absorbents minimizes aggregation associated with storage and transportation. Much higher surface area per unit mass of nanoparticles and per unit volume of the colloidal suspension than reported in the literature was obtained. The following trends in the colloidal concentration were common for the (w/o) microemulsion system and the heavy oil matrix. An optimum water to surfactant mole ratio, 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 precursor salt concentration. 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 by the microemulsions. Furthermore, the in-situ microemulsion approach developed in the first part was applied for in-situ preparation of effective H2Scgl colloidal absorbents within heavy oil matrix. H₂S_(g) is a by-product of in­situ heavy oil upgrading with potential negative impact on underground water. In this work, preliminarily evaluations of the effectiveness of the in-situ prepared colloidal iron oxide/hydroxide in heavy oil matrix for the absorption of H₂S_(g) was conducted successfully.