Titanium dioxide nanopowder is produced by a process, comprising: (a) reacting titanium tetrachloride and an oxygen containing gas in the vapor phase in a flame reactor, at a flame temperature of at least about 800° C. in the presence of (i) water vapor in an amount ranging from about 1000 to about 50,000 parts per million, based on the weight of titanium dioxide under production, (ii) a diluent gas in an amount greater than about 100 mole percent based on the titanium tetrachloride and oxygen containing gas and (iii) a nucleant consisting essentially of a cesium substance wherein the cesium substance is present in an amount ranging from about 10 to about 5000 parts per million, based on the weight of the titanium dioxide under production, the pressure of reaction being sufficient to form titanium dioxide nanopowder.