Boron nitride nanotubes (BNNTs) were synthesized from the reaction of ammonia gas with a powder mixture of boron and iron oxide in a tubular reactor connected to a mass spectrometer for the on-line chemical analysis of the reactor outlet stream at different temperatures and inlet gas compositions. XRD results showed that hexagonal and rhombohedral boron nitrides, iron, boron oxide, and iron boride were the solid phases formed in these materials depending on the reaction temperature and gas composition. It was observed that the crystallinity of the product increased with an increase in temperature. The synthesized materials exhibited Type II isotherms. The chemical analysis revealed that the only reaction taking place in the gas phase was the decomposition reaction of the ammonia gas and BNNTs were produced by the reaction of nitrogen formed from the decomposition reaction with FexBy formed from the reaction of B with Fe2O3. (C) 2012 Elsevier B.V. All rights reserved.