Mixed Ta2O5-containing SiO2 particles, 6-14 nm in diameter, with closely controlled refractive index, transparency, and crystallinity are prepared via flame spray pyrolysis (FSP) at production rates of 6.7-100 g h(-1). The effect of precursor solution composition on product filler (particle) size, crystallinity, Ta dispersity, and transparency is studied using nitrogen adsorption, X-ray diffraction, optical microscopy, high-resolution transmission electron microscopy (HRTEM), and diffuse-reflectance infrared Fourier-transform spectroscopy (DRIFTS). Emphasis is placed on the transparency of the composite that is made with Ta2O5/ SiO2 filler and dimethylacrylate. Increasing TA(2)O(5) crystallinity and decreasing Ta dispersity on SiO2 decreases both filler and composite transparencies. Powders with identical specific surface area (SSA), refractive index (RI), and Ta2O5 content (24 wt.-%) show a wide range of composite transparencies, 33-78%, depending on filler crystallinity and Ta dispersity. Amorphous fillers with a high Ta dispersity and an RI matching that of the polymer matrix lead to the highest composite transparency, 86%. The composite containing 16.5 wt.-% filler that itself contains 35 wt.-% Ta2O5 has the optimal radiopacity for dental fillings.