Tungsten trioxide is a semiconductor with distinct applications in gas sensors, catalysis, batteries and pigments. As such the transition between its different crystal structures during its annealing are of interest, especially for sensor applications. Here, WO3 nanoparticles with closely controlled crystal and grain size (9-15 nm) and phase composition are made by flame spray pyrolysis and the formation of different WO3 phases during annealing is investigated. Most notably, the dynamic phase transition and crystal size evolution of WO3 during heating and cooling is monitored by in situ X-ray diffraction revealing how metastable WO3 phases can be captured stably. The effect of Si-doping is studied since it is used in practise to control crystal growth and phase transition during metal oxide synthesis and processing. Finally the influence of annealing on the WO3 sensing performance of NO, a lung inflammation tracer in the human breath, is explored at the ppb-level. (C) 2014 Elsevier Ltd. All rights reserved.