The formation of ultrafine and nanosized particles of multicomponent catalysts of the Me (x) O (y) /Al2O3 and Me-2x S (y) /Al2O3 types (Me = Mo, Ni, Co, Fe) has been studied. Their samples have been synthesized by the thermal decomposition at T similar to 240A degrees C and P = P (atm) of reverse microemulsions (MEs) with precursors in the aqueous phase. The ME dispersion medium has been hydrocarbons containing synthetic (AOT, Span 80) or native (resins, asphaltenes, polycyclic aromatics) stabilizers. It has been shown that the mean diameter, structure, and morphology of the two- and three-component ultrafine and nanosized particles synthesized depend on the precursor introduction mode, ME composition, and microemulsion treatment conditions. It has been found that for the synthesis of spherical binary nanoparticles with the core-shell structure, the sequential introduction of precursors is preferable when the particle core phase (Al2O3) is synthesized in the first step and then a component that inevitably becomes the shell (Me (x) O (y) , Me (x) S (y) ) is generated.