The problem of the reversibility of paranatrolite-tetranatrolite transformation is a key problem in understanding the paragenesis of the natrolite group zeolites. Two paranatrolite samples of different chemical composition were studied by X-ray powder diffraction and thermogravimetry. It was found that the existence of a particular phase depends on the water vapor pressure in an ambient atmosphere. High-potassium paranatrolite from the Khibiny massif, Kola Peninsula, Na1.90K0.22Ca0.06[Al2.24Si2.76O10]·3.1H2O, is stable at 25 °C and air humidity of about 70 %. Upon heating, the sample loses some of the water content and transforms into tetranatrolite. At 38 °C it consists of a pure tetranatrolite phase. The reverse tetranatrolite-paranatrolite transformation occurs upon cooling the sample to room temperature. The recovery of the paranatrolite phase proceeds even after heating to 300 °C with a 60 % water loss. A high-calcium sample from Mont Saint-Hilaire, Quebec, with approximate formula Na1.59Ca0.32Sr0.02[Al2.35Si2.65O10]·nH2O, had significantly lower stability. It consisted of a mixture of paranatrolite and tetranatrolite in ambient conditions. Upon heating, the sample already consisted of a pure tetranatrolite at 31 °C. After keeping the sample for one day under normal conditions a two-phase mixture close to the initial sample was restored. The sample wetted by water immediately transformed into paranatrolite. A lower stability of high-calcium paranatrolite as compared with the high-potassium sample may be explained by the difference in the configuration of ionic-molecular assemblage and, presumably, by a higher water content.