In this contribution, experimental data are presented on the solubilities and partition coefficients of two semi-synthetic antibiotics (amoxicillin, ampicillin) and their precursors (D-phenylglycine, D-(p -hydroxy)phenylglycine, 6-aminopenicillanic acid) in water + 1-butanol mixtures at a constant temperature of 298 K. In the aqueous phase, the solubility of amoxicillin and ampicillin increases with increasing butanol concentration. For the other components, the solubility decreases. In the organic phase, the solubility of all components is less than that in the aqueous phase. The partition coefficients are approximately constant for all components and less than unity, indicating the higher solubility in the aqueous phase. No influence of the solutes on the phase behavior of the solvent system is detected. The phase behavior is described with the g(ex) model, as suggested by Gude et al. (Gude,M. T.; Meeuwissen, H. H. J.; Van der Wielen, L. A. M.; Luyben, K. Ch.A. M. Ind. Eng. Chem. Res. 1996, 35, 4700. Gude, M. T.; Van der Wielen, L. A. RI.; Luyben, K. Ch. A. M. Fluid Phase Equilib. 1996, 116, 110), in its original and in a modified version. This model, based on an excess solubility approach, contains a single adjustable interaction parameter that correlates with the hydrophobicity of the solute. The description by the original model is reasonable but cannot reproduce the phase behavior in the aqueous phase of the systems containing amoxicillin and ampicillin. The modified version of the model gives a better representation of the experimentally found phase behavior. Unfortunately, neither model describes solubilities and partitioning simultaneously in a quantitatively correct fashion.