Thermodynamic models provide the basis for the design of biochemical processes. However, for biological systems predictive or universally-valid models still do not exist in the literature. In this thesis thermodynamic properties of aqueous solutions with biological impact are considered, which may contain also alcohols (methanol, ethanol) as further solvents and solutes such as electrolytes (salts, acids, bases), amino acids/peptides, sugars, and osmolytes. Prior to the modeling of these systems, a consistent and broad experimental data basis was established in a first step. Appropriate experimental data for model-parameter estimation especially are activity coefficients as they are a direct measure for the molecular interactions in mixtures. On top of these values, also solution densities and solubilities of the binary systems osmolyte/water, amino acid/water, salt/alcohol and of the (qua)ternary solutions osmolyte/salt/water, amino acid/salt/water, and amino acid/amino acid(s)/water were measured for broad both temperature and concentration ranges, respectively. This provides the basis for applying a thermodynamic model. In this work the electrolyte Perturbed-Chain Statistical Association Theory (ePC-SAFT) developed by Cameretti et al.  in 2005 was used for modeling biological solutions. Components which form hydrogen bonds (e.g. water, alcohols, amino acids, or osmolytes) are described with five pure-component parameters whereas strong electrolytes (e.g. Na+ in NaCl) possess two ion-specific parameters only. This allows for a quantitative description of thermodynamic properties of electrolyte/water , electrolyte/alcohol , and biomolecule (amino acid, osmolyte, sugar)/water [4,5] solutions. Besides fully dissociated electrolytes or amino acids which are exclusively present as neutral zwitterions, also weak electrolytes and charged amino acids were considered. Modeling such systems succeeded by implementing respective dissociation equilibria within ePC-SAFT which explicitly account for the prevailing pH-value. Based on these binary solute/solvent systems, densities, activity coefficients, and solubilities in (qua)ternary solutions (salt/water/alcohol(s) , amino acid/salt/water, amino acid/amino acid(s)/water [4,6]) could be predicted with ePC-SAFT, i.e. additional fitting parameters were not required.