Zeolite H−ZSM-5 with a SiO2/Al2O3 ratio of 25 was evaluated for its ability to remove ethylene oxide (EtO) from streams of air at 25 °C and between 15 and 90% relative humidity (RH). The primary mechanism for the removal of ethylene oxide using H−ZSM-5 involves a catalyzed hydrolysis reaction initiated by the adsorption of EtO onto a Brønsted acid site. Secondary addition reactions leading to the formation of polyglycols are also occurring. The reaction is poisoned by the accumulation of reaction products within the pores of the zeolite, which ultimately leads to elution of EtO from the filter bed. The ability of H−ZSM-5 to remove EtO decreased significantly as the relative humidity increased from 50 to 90%. For example, the EtO breakthrough time decreases from 220 min at 50% RH to 57 min at 90% RH. The decrease in the EtO breakthrough time over the stated RH range is attributed to the increased hydration of the Brønsted acid site. As the extent of Brønsted acid site hydration increases, the reactivity of the site is reduced, leading to a reduction in the EtO breakthrough time.