The following four papers are a representative subset of the talks that were given during a lively session at the 1989 Winter Meeting of the Mineralogical Society (19 and 20 December), organised by us on behalf of the Metamorphic Studies Group on the theme of ‘Geothermometry and Geobarometry’. The talks at the meeting fell broadly into three groups: those dealing with fundamental problems of obtaining the requisite compositional and thermodynamic data, case studies of the applications of thermometers and barometers, and discussions of the limitations of the approach imposed by kinetic factors.

The first group of talks included contributions by T. J. B. Holland and J. Lieberman (co-authors M. Engi and R. G. Berman) dealing with perhaps the major hurdle in the way of reliable geothermometry and geobarometry: the question of activity-composition relationships for natural minerals. Holland took the examples of the pyroxenes and the plagioclase feldspars and showed, respectively, how Landau theory could be utilised to take account of ordering, and how Darken's quadratic formalism can take account of a structural break.