The perturbed-chain statistical association theory (PC-SAFT) is applied to simultaneously describe various thermodynamic properties (density, vapor-pressure depression, activity coefficient, solubility) of aqueous solutions containing an amino acid or an oligopeptide. The 28 organic compounds considered within this work are glycine, alanine, serine, proline, hydroxyproline, valine, leucine, arginine, lysine, threonine, asparagine, tyrosine, histidine, cysteine, methionine, aspartic acid, glutamic acid, α-ABA, α-isoABA, β-ABA, γ-ABA, α-AVA, γ-AVA, diglycine, triglycine, dialanine, Gly-Ala, and Ala-Gly. If not yet available in literature, amino-acid solubility data and activity coefficients were determined experimentally. To prove the predictivity of PC-SAFT, osmotic coefficients in aqueous solutions containing two amino acids (glycine/valine and alanine/valine) were measured and predicted without applying any additional model parameters.