We have investigated the effect of K loadings on the formation and the decomposition of KNO 3 over K 2 O/Al 2 O 3 , and measured NO x storage performance of Pt–K 2 O/Al 2 O 3 catalysts with various potassium loadings. After NO 2 adsorption on K 2 O/Al 2 O 3 at room temperature, ionic and bidentate nitrates were observed by Fourier transform infra-red (FTIR) spectroscopy. The ratio of the former to the latter species increased with increasing potassium loading up to 10wt%, and then stayed almost constant with additional K, demonstrating a clear dependence of loading on potassium nitrate formed. Although both K 2 O(10)/Al 2 O 3 and K 2 O(20)/Al 2 O 3 samples formed similar nitrate species identified by FTIR obtained after NO 2 adsorption, the latter has more thermally stable nitrate species as evidenced by FTIR and NO 2 temperature programmed desorption (TPD) results. With regard to NO x storage-reduction performance of Pt–K 2 O/Al 2 O 3 samples, the temperature of maximum NO x uptake (T max ) is 573K up to a potassium loading of 10wt%. As the potassium loading increases from 10wt% to 20wt%, T max shifted from 573K to 723K. Moreover, the amount of NO uptake (38cm 3 NO x /gram of catalyst) at T max increased more than three times, indicating that efficiency of K in storing NO x is enhanced significantly at higher temperature, in good agreement with the NO 2 TPD and FTIR results. Thus, a combination of characterization and NO x storage performance results demonstrates an unexpected effect of potassium loading on nitrate formation and decomposition processes; results important for developing Pt–K 2 O/Al 2 O 3 as potential catalysts as high temperature NO x storage-reduction applications.