The mechanism of CH3I adsorption and thermal decomposition by a Ag/Y sorbent with 23 wt % of silver and a Si/Al ratio of 2.5 was investigated using two different spectroscopic techniques. On the one hand, DR-UV-vis spectroscopy was used to monitor the evolution of silver species and their transformation to silver iodide AgI clusters during increasing exposure to gaseous methyl iodide and upon heating under dry and wet atmospheres. On the other hand, the evolution of adsorbed organic species was investigated under similar conditions by DRIFTS. The latter data were correlated with the evolution of gaseous products to elucidate the mechanistic pathways involved in the trapping and catalytic decomposition of CH3I. © 2016 American Chemical Society.