Surface tension of aqueous sodium chloride solutions has been measured in the concentration range 0M £C_NaCl £3M by the Du Nouy ring method. The surface tension of the NaCl solutions increases linearly with increasing salt concentration in good agreement with reported results. Without pre-heating- the NaCl solid sample to 520 ^oC- the surface tension displays a time dependent at a concentration of the 3 M. With pre-heating- the sample at the mentioned temperature- the surface tension stays constant with time du to removal of impurities. This simple experiment indicates the importance of heating in the measurement of surface tension of salt solutions.