Sub-stoichiometric WO 3- x has provided an alternative platform to investigate oxygen vacancies in gas sensors based on metal-oxides. We present an experimental study on the influence of sub-stoichiometric WO 3- x phase upon gas sensing performance. High-quality WO 3- x nanostructures with several x values (WO 3 , W 19 O 55 , W 5 O 14 , W 18 O 49 ) were synthesized and used to fabricate H 2 S gas sensors. Temperature programmed desorption of oxygen (O 2 -TPD) shows that oxygen absorption behaviors of the as-prepared WO 3- x nanostructures are affected by oxygen vacancies, which played a critical role in the detection of H 2 S at varying temperature range. We find that oxygen vacancies in sub-stoichiometric WO 3- x facilitate the ionosorption process and in turn enhance the performance of the gas sensor.