Abstract: Hydroquinone present as impurity with calcium 2,5-dihydroxy phenyl sulfonate was determined by FI-spectrophotometry with online extraction with ethyl ether.A manifold of online extraction incorporated with the FI-spectrophotometric unit was designed and fabricated especially for this purpose.Hydroquinone.was extracted into ether and its absorbance was measured at its absorption maxima of 297.5 nm in the organic phase,while the interference of the matrix compound,calcium 2,5-dihydroxy phenyl sulfonate,which is insoluble in ether,was eliminated.Linear relationship was kept between absorbance and concentration of hydroquinone in the range of 3.5×10-6～5.0×10-4mol·L-1 with a correlation coefficient of 0.999 4.Detection limit of the method was found to be 2.53×10-6mol·L-1.This method has been applied to the analysis of 3 samples from different batches of raw materials,the RSD′s (n=5) of the analyt.results obtalned were in the range of 0.69%-1.65%,and the results were also in consistency with the results obtained by the pharmacopeia method.