Densities, rho, and speeds of sound, u, of flucloxacillin sodium in water and aqueous NaCl solutions were measured at T = (298.15, 303.15, 308.15, 313.15, 318.15, and 323.15) K and atmospheric pressure using a high precision vibrating U-tube digital density and sound velocity analyzer (DSA 5000, Anton Paar, Austria). From the experimental data, apparent molar volume, phi(v), apparent molar adiabatic compressibility, phi(k), limiting apparent molar volume, phi(0)(v), limiting apparent molar adiabatic compressibility, phi(0)(k), and critical micelle concentration of flucloxacillin sodium were calculated. The limiting apparent molar volumes, phi(0)(v), were used to calculate Heplers constant. The results were interpreted in terms of solute-solvent and solute-solute interactions and structure making/breaking ability of the solute in the aqueous environment.