In order to investigate the mechanism of the photochromic effect in yttrium oxy-hydride (YO_xH_y) thin films, Doppler broadening positron annihilation spectroscopy (PAS) was applied to probe the electronic structure and the presence of vacancies in YO_xH_y and related materials as a function of composition, UV illumination and thermal annealing. The Doppler S and W parameter depth profiles of a series of Y, yttrium di-hydride YH_1.9+δ and Y₂O₃ thin films show strong systematic changes caused by the distinct differences in electronic structure of the metals Y, YH_1.9+δ and the wide band gap insulator Y₂O₃. The Doppler broadening parameters of photochromic YO_xH_y (a semiconductor with a band gap of ~2.6eV) are intermediate to those of YH_1.9+δ and Y₂O₃. In order to probe the nanostructural changes related to the photochromic effect, the S parameter of YO_xH_y was monitored during in-situ UV illumination. A small but systematic increase of the S parameter was observed, possibly induced by generation of cation mono-vacancies or small vacancy clusters involving generated anion vacancies. The changes did not relax during bleaching under dark conditions, showing that the structural changes are not directly responsible for the photochromic mechanism. For temperatures above around 90°C, thermal annealing leads to a substantial increase in the Doppler S parameter, pointing to the formation of vacancies by local removal of hydrogen. Simultaneously, the optical band gap increases, consistent with an increase in O:H ratios.