The morphology of flame-generated silica and titania aggregates is characterized by small-angle X-ray scattering(SAXS). Nearly all these powders display mass-fractal morphologies, which are composed of ramified aggregates of nanoscale primary particles. Primary particle size, aggregate size, fractal dimension, and specific surface area are obtained from this analysis. The mass-fractal dimension varies from 2.5 to 1.6 for flame generated silica and titania aggregates in single and double diffusion flame reactors. However, titania powders made in a single diffusion flame reactor appear as nonaggregates and nonfractals. Silica powders synthesized with an imposed electric field in a laminar, premixed flame reactor are mass fractals with narrowly confined fractal dimensions from 1.5 to 1.9 regardless of aggregate size.