This invention relates to a process for treating an exhaust gas stream from an engine, especially during cold start. The process involves a molecular sieve bed over which the cold exhaust is flowed before flowing over a catalyst bed. Pollutants such as hydrocarbons are adsorbed on the molecular sieve bed. When the molecular sieve bed reaches a temperature of about 150.degree. C., the pollutants are desorbed from the adsorbent bed and converted by the catalyst to innocuous compounds. The molecular sieves used in the process are those that selectively adsorbs pollutants, e.g., hydrocarbons versus water, and are hydrothermally stable.