Abstract: Microwave heating was adopted for the desulfonation of straight chain sodium alkyl benzene sulfonates in the GC determination of mean relative molecular mass.The sample was digested in the microwave oven with H3PO4 in a tightly closed PTFE vessel at 150 ℃ for 10 min and at 180 ℃ for 5 min successively with out put power of 300 W.It was proved that the desulfonation method proposed was superior to the pyrolytic method given in GB/T 5178-1985,in respect of simplicity in operation,requiring less time of treatment and guarantee to give accurate and reliable results.In the GC determination,HP-5 quartz capillary chromatographic column (25 m×0.32 mm,0.25 μm) and hydrogen flame ionization detector were used.