Process for producing bases by condensing an aliphatic aldehyde and/or ketone with ammonia in gas phase in the presence of a catalyst, characterized by using a catalyst which is obtained by modifying a zeolite having a proportion of Si to Al, Fe and/or Ga of about 12 to about 1,000 and a constraint index of about 0.8 to about 12 with at least one metal ion or metal compound, the metal being selected from the group consisting of thallium, lead and cobalt.