PURPOSE: To remove contaminants effectively both at the starting period when an engine is cold and after warmed up without using a by-pass valve in an exhaust gas conversion system.

CONSTITUTION: In this exhaust gas conversion system 40, an exhaust gas in undiverted flow is brought into contact with a zeolite 42 suitable for adsorption of hydrocarbons and with a main catalyst 46 suitable for conversion of NOX, CO, and hydrocarbons to innocuous products. Conversion occurs when the temp. of the main catalyst 46 is suitable for conversion. The zeolite is selected from the group consisting of Y zeolite, Beta, ZSM-5, and combinations thereof, and has a SiO2 to Al2O3 molar ratio of no greater than about 200. The zeolite is ion exchanged with a metal selected from the group consisting of rare earth, chromium, and combinations thereof.