Provided are: a hydrocarbon adsorbent capable of adsorbing hydrocarbons, storing the adsorbed hydrocarbons up to a relatively high temperature, and desorbing the adsorbed and stored hydrocarbons at a relatively high temperature; an exhaust gas purifying catalyst composition using the same; an exhaust gas purifying catalyst; and a method for treating an exhaust gas. The hydrocarbon adsorbent comprises a zeolite having an MRT-type framework structure. The hydrocarbon adsorbent comprises a small-pore zeolite having a total desorption amount ZD.sub.1 of propylene desorbed at 50 C. or higher and lower than 300 C. being 3.5 mmol/g or less and a total desorption amount ZD.sub.2 of propylene desorbed at 300 C. or higher and 500 C. or lower being 0.5 mmol/g or more, per 1 g by mass of the small-pore zeolite, when adsorbing propylene at 50 C. and then heating from 50 C. to 500 C. under the condition of 10 C./min by a temperature-programmed desorption method.

An adsorbent for passive NO.sub.x adsorption includes a small pore zeolite having an eight-ring framework structure and a non-noble metal ion doped in the framework structure. An exhaustion system comprising the adsorbent and a method for preparing the adsorbent are also addressed.