A process for precisely controlled synthesis of nanoparticles with a broad range of sizes, morphologies, and phase compositions is presented. This is achieved by a rapid quenching of the entire flame aerosol in a critical-flow nozzle placed above and into the flame. This process is evaluated for synthesis of titania nanoparticles by oxidation of titanium-tetra-isopropoxide (TTIP) in a methane/oxygen coflow diffusion-flame reactor. Precise control of phase composition from 97 to 5 wt. % anatase (and the balance rutile) and average primary particle diameter from 5 to 60 nm is possible by positioning the quenching nozzle at the desired heights above the burner and controlling gas and precursor flow rates. The nozzle quenching also reduces the degree of agglomeration of the product particles. An operation diagram shows that the primary particle diameter and the phase composition can be independently controlled, making anatase or rutile nanoparticles with high or low specific surface area.