Summary: Mesolite is an independent species, isostructural, but not isomorphous with natrolite and scolecite. Five new analyses have been made. The unit cell, of dimensions a 56·7, b 6·56, c 18·44 Å., β 90°0′, contains Na16Ca16Al48Si72O240.64H2O. The space-group is C23 but there is a marked approximation to the orthorhombic space-group C2v19. Goniometric measurements are in excellent agreement with the X-ray data, and show a marked development of vicinal pyramids. The optical properties, vapour-pressure, and base-exchange of mesolite have been studied in considerable detail. It has been found possible to make a ready distinction between mesolite and natrolite by X-ray methods, and it has been shown that by the action of fusions of potassium thiocyanate, lithium nitrate, and silver nitrate on mesolite, compounds identical with similar derivatives prepared from natrolite may be obtained; these have the natrolite structure. The vapour-pressure measurements have been checked by a direct determination of the heat of hydration of partially dehydrated mesolite. Mesolite has been observed on specimens from eight localities new for the species.