Summary: Natrolite is shown by nine new analyses and new X-ray measurements to have a constant Si/Al-ratio and a unit-cell formula of Na16Al16Si24O80.16H2O, in agreement with previous results. Na2→Ca replacement may occur up to about 4, and Na→K replacement up to about 2 atoms per unit cell. Natural etch-figures on natrolite from Benallt, Carnarvonshire (a new locality), show the c-axis to be polar. A detailed study of the optical properties of natrolite has been made. Some observations have been made on the effects of partial dehydration on the optical properties. The vapour-pressure has been studied at various temperatures and degrees of hydration, and several peculiarities observed. The heat of hydration has been measured and shown to be in agreement with the vapour-pressure work. Some experiments have been carried out on the base-exchange of natrolite. An X-ray study of natrolite and metanatrolite, together with the vapour-pressure observations, suggests that natrolite is really monoclinic, with space-group C23, the symmetry-axis being parallel to the elongation (the usual c-axis). But there is very marked pseudosymmetry, the symmetry very closely approaching that of the orthorhombic space-group C2v19, of which C23 is a sub-group.