Zinc oxide-titanium dioxide composite nanoparticles are prepared using a supercritical water reactor-in-series apparatus. Zinc oxide particles are produced from zinc acetate in the first reactor, and the particle suspension is fed to the second reactor where titanium oxide is formed from a titanium tetrachloride solution yielding ZnO-TiO2 composite nanoparticles. Reverse reactor order is also studied. Rapid and continuous production of nanosized composite particles is achieved in this reactor-in-series apparatus at 400 degreesC and 245 atm. Dynamic light scattering is used to determine the particle size, transmission electron microscopy is used to determine the particle morphology, and energy-dispersive X-ray spectroscopy is used to determine the composition.