The effect of quench conditions on the thickening and thixotropic ability of fumed silica was studied. The powder was generated by rapidly quenching a vapour stream containing SiO(g) with steam. Silicon monoxide vapour was produced by decomposing quartz in a transferred arc reactor using an Ar/NH3 thermal plasma. The parameters investigated included pre-quench temperature (1100 - 2100 K), and supersaturation ratio (1 - 10(7), quench rate (17000 - 75000 K/s), and quench stoichiometry (27 - 84), which was defined as the molar ratio of steam to SiO. The thickening and thixotropic ability of the powders increased with increasing quench rate and decreasing pre-quench supersaturation. This was due to an increase in specific surface area. Use of large H2O/SiO ratios caused a high degree of surface hydroxylation.