A method of treating a cold-start engine exhaust gas stream comprising hydrocarbons and other pollutants, the method comprising: flowing the exhaust gas stream over a molecular sieve bed, the molecular sieve bed comprising an alkali metal cation-exchanged molecular sieve having intersecting 10- and 12-membered ring pore channels, to provide a first exhaust stream; flowing the first exhaust gas stream over a catalyst to convert any residual hydrocarbons and other pollutants contained in the first exhaust gas stream to innocuous products to provide a treated exhaust stream; and discharging the treated exhaust stream into the atmosphere.