A simple new characterization method of ultra-microporous structures was developed by measuring pressure change through gas diffusion in constant volume around room temperature. Several gas species such as He, Ne, Ar, N 2 , and CH 4 , which had different molecular size each other, were utilized as molecular probes for detecting effective micropore volume for gas diffusion under a room temperature in gas phase. The feature of this characterization method was that observed micropore volume for

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... diffusion depended on the probing gas molecular size, and it enabled the quantitative estimation of effective micropore volume for each probing molecule. Zeolite Y (FAU) and ZSM-5 (MFI), which were known to have regular micoporous structures, were adopted as a standard sample for verifying the mulita-gas diffusion method. The conventional N 2 adsorption at 77 K was also employed to characterize microstructures of those zeolites and both the validity and superiority of the proposed multi-gas diffusion method were examined. The volume of major pores of FAU, which is called α-cage whose size is about 1.3 nm, was also detected by diffusion of larger molecules such as Ar and N 2 . Ordinal N 2 adsorption method can give the pore size and volume of only α-cage, however, a bi-modal porous structure of FAU comprised of α-cage and SOD (βcage) was successfully detected by diffusion of smallest He molecule. For the case of ZSM-5, all probing gas molecules showed similar but slightly different micropore volumes due to the different diffusing molecular size. Adsorption enthalpy was also reasonably estimated by the technique.