By using a specially designed and constructed isopiestic apparatus, we measured the osmotic coefficients at 313.2K for the NaOH-NaAl(OH)4-H2O system with the total alkali molality, mNaOHT (mNaOH+mNaAl[OH]4), from 0.05mol/kg H2O to 12mol/kg H2O and αK (mNaOHT/mNaAl(OH)4) from 1.64 to 5.53. The mean standard deviation of the measurements is 0.0038. Several sets of the Pitzer model parameters for NaOH-NaAl(OH)4-H2O system were then obtained by regressing the measured osmotic coefficients with the Pitzer model and the Pitzer model parameters for NaOH(aq). One set of the results is as follows: β(0) NaOH: 0.08669, β(1)NaOH: 0.31446, β(2) NaOH: β0.00007367, C(1)NaOH: 0.003180, β(0) NaAl(OH)4: 0.03507, β(1) NaAl(OH)4: 0.02401, C(1) NaAl(OH)4: β0.001066, θOH-Al(OH)4-: 0.08177, ψNa+OH-Al(OH)4-: -0.01162. The mean standard difference between the calculated and the measured osmotic coefficients is 0.0088. With the obtained Pitzer model parameters, we calculated the values of K1=(γNaAl(OH)4, cal2-mAl(OH)4-, exp)/(γNaOH, cal2-mOH-, exp) for the gibbsite solubility. The results show that the obtained Pitzer model parameters are reliable, and the relative error of the calculated activity coefficients should be <2.1%. We also compared the calculated gibbsite solubility data among several activity coefficients models over a range of mNaOHT at various temperatures. The comparison indicates that our activity coefficients model may be approximately applied in the ranges of temperature from 298.2 to 323.2 K and mNaOHT from 0 to 8 mol/kg H2O. We also calculated the stoichiometric activity coefficients of NaOH and NaAl(OH)4 and the activity of H2O for the NaOH-NaAl(OH)4-H2O system, and these calculations establish their variations with mNaOHT and αK. These variations imply that the strengths of the repulsive interactions among various anions are in the following sequence: Al(OH)4-Al(OH)4-<Al(OH)4-OH-<OH-OH-, and the attractive interaction between Al(OH)4 and H2O is weaker than that between OH and H2O. Copyright.