The solubility in aqueous fluids of peralkaline alkali aluminosilicate components has been determined in the 0.8 to 2.0 GPa and 1000 to 1300°C pressure and temperature range, respectively. Compositions were along the haploandesite join, Na 2 Si 4 O 9 –Na 2 (NaAl) 4 O 9 , with 0, 3, and 6 mol.% Al 2 O 3 , denoted NS4Ax, where x represents the mol.% Al 2 O 3 . The silicate solubility in aqueous fluid is in the 3 to 20 mol.% range for NS4, 2 to 13 mol.% for NS4A3, and 1.5–8 mol.% for NS4A6 with a linear and positive temperature-dependence and a nonlinear and positive pressure-dependence. The silicate solubility decreases with increasing Al 2 O 3 content. From stepwise regression, the pressure-, temperature-, and Al 2 O 3 -dependence of the solubility can be described with the expression: X silicate (mol.%)=1.9−1.3X Al2O3 (mol.%)+0.008T(°C)+13P(GPa)+7.3P 2 . Partial molar volume of H 2 O in the silicate-saturated fluids, V̄H 2 O fluid , is in the range 17 to 25 cm 3 /mol. The V̄H 2 O fluid was calculated with the assumption that the activity of dissolved silicate in aqueous fluid equals the mol fraction of dissolved silicate. Compared with the molar volume of pure H 2 O, V ° H2O , the V̄H 2 O fluid values are 10 to 15% lower at 0.8 GPa and as much as 15% higher than V ° H2O at 2.0 GPa. This volume difference diminishes as the system become more aluminous. For all compositions, the V̄H 2 O fluid is a linear and negative function of pressure with β[= −1/V 0 (∂V̄H 2 O fluid /∂P) T ] ∼ 0.125 GPa −1 for NS4. The β NS4A3 increases from 0.141 to 0.172 in the 1000° to 1300°C range, whereas β NS4A6 increases from 0.193 to 0.213 GPa −1 . The V̄H 2 O fluid is a positive and linear function of temperature with thermal expansion coefficients in the range 3–8 × 10 −4 K −1 . This thermal expansivity resembles that of pure H 2 O although the exact values and the range in values for pure H 2 O differ from 1/V 0 (∂V̄H 2 O fluid /∂T) P of silicate-saturated aqueous solution.