Densities of NaOH + NaAl(OH)(4) solutions were measured in isomolal series at ionic strengths (1) over the range of 1 <= 1/mol kg(-1) <= 6 and at total concentration ratios of 0 <= [Al(III)](T)/[Na+](T) <= 0.6 in the temperature range of 50 <= t/degrees C <= 90. A Pitzer model representing the volumetric properties of hypothetical pure sodium aluminate solutions was derived and incorporated into our thermodynamic model of synthetic Bayer liquor solutions consisting of the components NaOH + NaAl(OH)(4) + Na2CO3 + Na2SO4 + NaCl + NaF + Na2C2O4 (sodium oxalate) + NaHCOO (sodium formate) + NaCH3COO (sodium acetate) + H2O. The model calculations agree with experimental densities of twelve 7-component synthetic Bayer liquors to better than 0.1%.