Abstract A gas chromatographic method for the determination of formoterol ( RS, SR)-diastereoisomer, a process impurity, in formoterol fumarate was optimized and validated. The method involves silylation of formoterol fumarate with N-(trimethylsilyl)imidazole in N, N-dimethylformamide at room temperature in an autosampler vial to produce trimethylsilyl derivatives of the enantiomers prior to GC analysis. The optimized silylation and separation conditions, respectively, produced good yield and resolution of the analytes. The method appears to be convenient and fast, and permits accurate determination of ( RS, SR)-diastereoisomer in formoterol fumarate with adequate precision (R.S.D. = 3.0%, n = 9) and sensitivity (DL < 0.01%) when compared with the official liquid chromatographic limit test method of Pharmeuropa. The method was successfully applied to quality control of commercial formoterol fumarate for their ( RS, SR)-diastereoisomer contents.