The solubilities of analcime and clinoptilolite were determined in dilute, weakly alkaline, aqueous solutions below 300 degrees C and at vapor-saturated pressures. Analcimes used in this study were from Mont St. Hilaire, Quebec (an 1 , Si/Al = 2.02) and Wikieup, Arizona (an 2 , Si/Al = 2.55); clinoptilolite samples were from Castle Creek, Idaho (Si/Al = 4.50). The effects of alkali content (Na,K) on clinoptilolite solubility were determined by using cation-exchanged varieties of the Castle Creek material (cp 1 , cp 2 ). In neutral to weakly alkaline solutions, the dominant solubility-controlling reactions of these zeolites are NaAlSi 2 O 6 .H 2 O(an 1 ) + 5H 2 O(1) <--> Na (super +) + Al(OH) (super -) 4 + 2Si(OH) 4 (aq), Na (sub 0.85) Al (sub 0.85) Si (sub 2.15) O 6 . H 2 O(an 2 ) + 5H 2 O(1) <--> 0.85Na (super +) + 0.85Al(OH) (super -) 4 + 2.15Si(OH) 4 (aq), Na (sub 1.1) Al (sub 1.1) Si (sub 4.9) O 12 .3.5H 2 O(cp1) + 8.5H2O(1) <--> 1.1Na (super +) + 1.1A1(OH) (super -) 4 + 4.9Si(OH) 4 (aq), and K (sub 1.1) Al (sub 1.1) Si (sub 4.9) O 12 .2.7H 2 O(cp 2 ) + 9.3H 2 O(1) <--> 1.1K (super +) + 1.1Al(OH) (super -) 4 + 4.9Si(OH) 4 (aq). The logarithm of the equilibrium constants of these reactions were fit to the function: log K = A + BT + C/T + D log T. At 25 degrees C, log K 25 values for the Mont St. Hilaire analcime, Wikieup analcime, Na-clinoptilolite, and K-clinoptilolite are -16.1, -15.0, -26.5, and -28.1, respectively. These data were combined with the thermodynamic properties of the aqueous (aq) species Si(OH) 4 , Al(OH) (super -) 4 , Na (super +) , K (super +) , and liquid water (1) to determine standard Gibbs free energies of formation as a function of temperature. Values of Delta G 0f at 25 degrees C and 1 bar for the Mont St. Hilaire analcime and Wikieup analcime are -3089.2 and -3044.4 kJ/mol, respectively. The Delta G 0f values for hydrous Na-clinoptilolite and K-clinoptilolite, respectively, are -6267.9 and -6107.4 kJ/mol at 25 degrees C and 1 bar. The solubility data reported here, and results obtained from previous calorimetric studies, indicate that the aluminosilicate frameworks of analcime and clinoptilolite are stabilized by an increase in Al content.