Flux of ozone (O3) to indoor surfaces is a function of airflow and surface O3 reactivity. In addition to material composition, surface reactivity is affected by mass accretion due to particles and semi-volatile organic compounds adsorbed to surfaces. We investigate the impact of mass accretion on O3 reactivity using glass surfaces as a test material. Initially clean, inert glass surfaces were deployed to field sites for 0-60 days and collected weekly. Average mass accretion rates were 18.5 mg/(m2 d) and 10.6 mg/(m2 d) in a naturally ventilated (NV) and mechanically ventilated (MV) space, respectively. Reaction probabilities generally increased with time deployed. Regression analysis shows accreted mass removed 4.3 nmol O3/mg and 2.4 nmol O3/mg for the NV and MV space, respectively. A study of volatile organic byproducts formed from O3-accreted mass reactions is currently underway.