Although germanic acid is an important compound in many engineering and natural aqueous environments, its exact stoichiometry, structure and stability are insufficiently known. To fill this gap, we combined theoretical quantum chemistry simulations of Ge speciation and structure with a recently developed equation of state for aqueous neutral species, applied to available experimental GeO2(s) solubility measurements. Results allow generation of a consistent set of thermodynamic properties of Ge (OH)(4)(aq) over a large T-P-fluid density range (298-900 K, 0.1-300 MPa, and 0.01-1.0 g cm(-3)). These properties enable quantitative predictions of Ge transport and its fractionation from similar elements (e.g. Si) in aqueous vapour liquid and supercritical fluid systems. (C) 2015 Elsevier B.V. All rights reserved.