To provide an antioxidant compsn. which exhibits a high antioxidant action in a lubricating oil without corroding a sealant and a copper material by incorporating, into the compsn., a thiophenol obtd. by reacting a chlorine-free sterically hindered phenol with a sulfur source and an alkali metal hydroxide in 1-propanol.

This compsn. contains at least one thiophenol of the formula (wherein R is alkyl; R1 is H or alkyl; one of Z, Z1, Z2 and Z3 is OH and the rest are each H; x is 1-8; and y is 0-2) as an antioxidant. A substantially liq., highly sulfidized sterically hindered thiophenol is prepd. by reacting a sterically hindered phenol mixture (e.g. 2,4-diisopropylphenol) with elemental sulfur in the presence of an alkali metal hydroxide accelerator and 1-propanol and washing the resultant product with water. Thus is prepd. a chlorine-free antioxidant having a high solubility in a lubricating oil, which can be effectively used in motor oils including a low-phosphorus-content one.