It is known, that pectolite exhibits the lowest deuterium content among all the hydrominerals. This is due to pecularities of its structure. In present work the kinetics of extraction of crystallic water from pectolite and from muscovite is examined. In the first case preferently “heavy” water (i.e. enriched in its deuterium content) is expelled from the mineral and in the second case – “light” water. An activation energy of 60 keal/mole may be attributed to dehydratation of both the minerals. Experiments show, that delta D in synthetic pectolites does not depend on the isotopic composition of surrounding water.