[en] In the nuclear spent fuel reprocessing plants, solid filters are the last device to trap iodine-129 in the gaseous effluents before atmospheric discharge. Molecular iodine is fixed by chemisorption on silver nitrate deposited in the porosity of a catalyst support. Within the framework of the researches on enhanced separation of LLFP, a program was undertaken aiming at identifying the trap properties controlling the iodine retention. Two industrial substrates were investigated to determine the influence of their physicochemical characteristics, successively on the reactive deposit and iodine trapping. The isotherms showed two quite distinct successive stages: chemisorption prevails as long as the silver nitrate is not completely converted and, then only, physi-sorption takes place controlled by the porous texture. The kinetics can be treated according to the LDF model which gives access to the kinetics constant for a given gas-solid equilibrium; the variation of this constant was assessed in a wide range of P/P₀. (authors)