The thermodynamic properties in aqueous solution of the amphiphilic anticoagulant drug warfarin sodium in the temperature range (20 to 50) degrees C were determined. The surface behavior and the critical micelle concentration (cmc) at 20 degrees C were determined by surface tension measurements. The electrical conductivity was measured as a function of the concentration at various temperatures. The Phillips and Williams methods were applied to obtain the critical micelle concentration from these data. Both methods lead to similar values of the cmc. Measurements of density in the range of concentrations from (0.005 to 0.400) mol center dot kg(-1) were carried out at (20, 30, 40, and 50) degrees C. From these results, the apparent molal volumes have been calculated as a function of concentration and temperature.