A powder sample of Li0.4WO3 was studied after exposure to air in steps up to a total exposure time of 71 days. Over this period, XPS spectra of the W 4f, O 1s and C 1s level were recorded. The spectra reveal the formation of a OH/CO3 layer rendering the powder insulating. Careful evaluation of the W 4f spectra suggests a single initial state picture in which the electron donated by Li is shared between W ions. We demonstrate how the loss of charge carriers by aging in air can be followed by the fitting parameters. Additionally, the effects of vacuum storage, inducing oxygen vacancies, and subsequent treatment with molecular oxygen are considered. (C) 2015 Elsevier B.V. All rights reserved.