Controlling nanocomposite composition and morphology is a vital step toward designing new and advanced materials. The current work presents the results of an experimental investigation of the use of mixed-phase reactants for the synthesis of nanocomposite materials. Gas-phase tetramethyltin was used as a precursor for tin dioxide (SnO2). Metal additives were introduced to the SnO2 synthesis system using solid-phase metal acetates as the precursors. The physical and chemical properties of the metal acetate reactants, the combustion environment and the nanocomposite materials were characterized in order to clarify the reaction processes important during synthesis of nanocomposite materials from these categories of mixed-phase reactants. X-ray diffractometry was used to determine composition and the crystalline structure of reactant and product materials. Scanning and transmission electron microscopy were used to examine particle morphology of reactant and product materials, and X-ray energy dispersive spectroscopy was used for elemental speciation. The results indicate the metal acetates are an excellent source of metal and metal oxide additives in a flame reactor. The metal acetates rapidly decompose and experience considerable restructuring, leading to metal additives with a range of microstructures in the SnO2 nanocomposites: from metal encapsulation in SnO2 to mixed aggregates. (c) 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved.