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.