A badge-type passive monitor was used to evaluate the effectiveness of four ozone trapping reagents for measuring O 3 concentrations in the air. These were sodium nitrite (NaNO 2 ), 3-methyl-2-benzothiazolinone acetone azine (MBTH), p-acetamidophenol (p-ATP), and indigo carmine. Experiments in an exposure chamber showed that only NaNO 2 and MBTH monitors gave sensitive and linear responses over realistic ranges of O 3 concentrations. When tested in ambient air, NaNO 2 and MBTH monitors with a single-layer diffusion barrier overestimated O 3 concentrations by a significant amount. This was largely canceled out in the NaNO 2 monitor by using a multi-layered diffusion barrier to combat wind turbulence effects. However it had almost no effect on the MBTH monitor, and it was found that NO 2 was a source of serious interference. We concluded that using the NaNO 2 monitor with an effective diffusion barrier can measure O 3 in ambient air with an accuracy of ±16%.