There is described a process for preparing metal oxide particles which are substantially free of coarse tail from an oxidizing agent and a vaporous metal reactant in a flow reactor; comprising, (a) directing a flow of the metal reactant into a contacting region of the flow reactor; comprising (a) passing a flow of oxidizing agent through a high temperature zone of the flow reactor to form a flow of hot oxidizing agent and directing the flow of the hot oxidizing agent onto the contacting region of the flow reactor at a flow condition sufficient to form a reaction stream comprising a flow of hot oxidizing agent, a flow of metal reactant and a diffusive flow of the hot oxidizing agent and the metal reactant, the temperature of the hot oxidizing agent being at least sufficient to initiate oxidation of the metal reactant in the diffusive flow; (c) passing the reaction stream into a reaction zone of the flow reactor, while simultaneously introducing a flow of an upper cooling fluid substantially coaxially with the reaction stream to form a fluid curtain which surrounds the reaction stream; (d) maintaining the fluid curtain while the metal oxide particles form within the diffusive flow of the reaction stream and until at least a major portion of the reaction stream is cooled to a temperature below the temperature at which metal oxide particles coalesce; and (e) separating the metal oxide particles from the reaction stream.