Abstract: The interaction of ammonia with Na-, NH₄-, Ca-, Cu-, and Mg- forms of two vermiculites has been investigated by infrared spectroscopy and chemical analysis. Both NH₄ ions and co-ordinated NH₃ are produced in the interlayer space in amounts which depend on the exchangeable cation and the particle size of the vermiculite. With the exception of that in Cu-vermiculite, the co-ordinated NH₃ is rapidly displaced by atmospheric water vapour. NH₄ ions are slowly decomposed at normal humidity and more rapidly and completely at high humidity. The co-ordinated NH₃ in Cu-vermiculite is converted to NH₄. Relative amounts of NH₄ and co-ordinated NH₃ are influenced by particle size, smaller particles favouring NH₄. The stabilities of both species to water vapour increase with particle size.