Simultaneous synthesis of H-2 and ZnO nanoparticles is investigated by steam-hydrolysis of Zn vapor in a hot-wall aerosol flow reactor with separated evaporation, cooling, and reaction zones. Superheated Zn vapor was carried by Ar into a tubular, quartz reactor where it was mixed and quenched by a superheated, equimolar H2O/Ar stream, resulting in Zn/ZnO nanoparticles and H-2. The Zn(g) vapor was generated by electric heating of a Zn crucible whose weight was continuously monitored and compared to H-2 production rate. The reactor was operated at I bar and 573-1273 K: above and below the Zn saturation vapor pressure suppressing and allowing, respectively, Zn aerosol formation by condensation taking place in parallel with the omnipresent ZnO formation by gas phase or surface hydrolysis of Zn. The process yielded up to 90% H-2 conversion and nanoparticles with Zn and ZnO mean crystallite sizes of 100 and 40 nm, respectively, containing up to 80 wt % ZnO. (c) 2006 American Institute of Chemical Engineers.