Amorphous silica (SiO2) ribbons were prepared by hydrolyzing SiCl4 at highly oriented pyrolytic graphite (HOPG) surfaces on which water had condensed. Exposure to humid air (>30% relative humidity (RH)) at room temperature caused water to condense selectively at hydrophilic step edges on an otherwise hydrophobic graphite surface. Upon exposure of this surface to SiCl4 vapor, a reaction with condensed water at step edges caused the formation Of SiO2 according to the reaction SiCl4 + 2H(2)O --> SiO2 + 4HCl. The shape and size of the SiO2 nanostructures varied with the RH: Below 20% RH, nanoparticles Of SiO2, aligned at step edges, were observed. For RH of 35-50%, continuous nano- and microribbons Of SiO2 were obtained. For higher RH, micron-scale silca particles and ribbons were both observed. At a RH of 35%, silica ribbons as small as 80 nm (width) x 20-40 nm (height) up to 500 mum in length could be prepared. Repetitive dosing of the HOPG surface in the humid environment with SiCl4 produced nano- and microribbons with a height and width that increased in proportion to square root of the SiCl4 dose.