The vapor-pressures, liquid densities and solubilities of aqueous amino acid and oligopeptide solutions were modeled with an equation of state based on PC-SAFT. The amino acids glycine (Gly), alanine (Ala), serine (Ser), proline (Pro), and valine (Val) and the oligopeptides (Gly)_n=1..5, and Ala(Gly)_n=1..4 were considered: Five pure component model parameters for each amino acid were fitted using experimental pvT, VLE, and SLE data. The oligopeptides were treated as co-polymers built up by the respective amino acids. Thus, no additional parameters had to be fitted for the prediction of pvT and VLE behavior of their aqueous solutions besides the segment number. The model is able to excellently reproduce the experimental density, vapor pressure and solubility data of aqueous amino acid and oligopeptide solutions. Keywords: amino acid solutions, pvT, VLE, SLE, equation of state, PC-SAFT