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.