Pt/zeolite catalysts have high CO selectivity for preferential oxidation of CO (PROX). To clarify this reason, characterization of Pt/ZSM-5 catalysts with high and low selectivity was performed. It was found that the catalyst with high selectivity consists of small Pt particles supported in ZSM-5 pores, but another one does of large particles locating outside. Chemical and/or physical molecular sieve effects in ZSM-5 must be responsible to the high selectivity. This concept can be applied to other metal/zeolite systems.