Abstract: A differential infrared method is described for the quantitative determination of ethoxy group in silicone oil by using the characteristic absorption of the ethoxy group at the band of 960 cm-1. The sensitivity of this method is about 0.1% and its accuracy is 0.015%. The phenyl groups in silicone oil can be determined by ultraviolet spectrophoto-metric method with 0.53% accuracy or by measuring the silicone content in it. The results obtained from the two methods have no noticable difference. The ultraviolet spectrophotometric method can also be used for determination of the relative ratio of the two different structures. in methylbenzyl silicone oil.