In this study, the partial molar volumes of L-serine and L-threonine in aqueous solutions of ammonium sulfate at (0.0, 0.1, 0.3, 0.7, and 1.0) mol.kg(-1) are reported between 278.15 and 308.15 K. Transfer volumes and hydration numbers were obtained, which are larger in L-serine than in L-threonine. Dehydration of the amino acids is observed, rising with the temperature and salt molality. The data suggest that interactions between ions and charged/hydrophilic groups are predominant, and by applying the McMillan and Mayer formalism, it was concluded that they are mainly pair wise. The combination of the data presented in this study with solubility and molecular dynamics data suggests a stronger interaction of the ammonium cation with the zwitterionic centers of the amino acids when compared to the interactions of those centers with the sulfate anion. ; This study was supported by Project PEst-C/EQB/LA0020/2011, financed by FEDER through COMPETE—Programa Operacional Factores de Competitividade, and by FCT—Fundação para a Ciência e a Tecnologia. This work was also co-financed by QREN, ON2 and FEDER (Project NORTE-07- 0162-FEDER-000050). The stay of P. C. Mota at ICT Prague was arranged within the ERASMUS Program.