Microporous solids such as zeolites find widespread use in the chemical industry as molecular sieves and catalysts. The scope of microporous materials has been extended by the development of aluminophosphates (AlPOs) and more recently by metal-organic hybrid materials (section 3.1.1). Zeolites can be thought of as inorganic analogues to biological enzymes. To rationalise their properties and predict new uses for specific materials it is necessary to determine the location of extra framework cations (e.g. Ca and K), guest molecules (including low-Z D and Li), the ordering of framework (Si/Al) atoms and accurate site occupancies. To date only 27 zeolite structures have been determined by single crystal neutron diffraction, several associated with the same parent zeolite, a small minority of the 139 known structure codes (framework topologies). Using the small crystal capability of LMX, key features of many more zeolites can be determined. For example: (i) the structure of stable hydrated intermediates during dehydration during catalyst activation, (ii) the location of protons at catalytic centres, (iii) the location of polarising cations of low atomic mass e.g. Li+ used in gas separation, (iv) the location and occupancy of guest species (catalysis and gas separation) and (v) the determination of as-synthesised materials showing the location of templating organic molecules, important for understanding the nature of host-guest interactions in the synthesis of new framework compounds. All of these can be determined in an in-situ manner, further making use of the relative transparency of environmental cells to neutrons.