Summary: Edingtonites from Old Kilpatrick and from Böhlet have been shown to be essentially identical, small differences being due to an appreciable calcium content in the former. The unit-cell formula of edingtonite is Ba2Al4Si6O20.8H2O. The cell dimensions have been determined for both Old Kilpatrick and Böhlet material which differ appreciably. In both cases there is good agreement between the goniometric and X-ray axial ratios. The optical properties of both Old Kilpatrick and Böhlet material have been studied. A new analysis of the latter gives results in agreement with the above formula. The vapour-pressure of Böhlet edingtonite has been determined at various temperatures and degrees of hydration, and checked by a direct determination of the heat of hydration. By base-exchange, thallous- and silver-edingtonites have been prepared, and also potassium and sodium compounds (pseudo-edingtonites), which have a unit cell approximately eight times as large as that of edingtonite. None of these products is identical with the corresponding base-exchange products from the mesotypes or from thomsonite, in conformity with their essentially different structures.