1. Infrared and ultraviolet spectroscopy and adsorption and thin-layer chromatography have been used for studying the chemical structure of the alkyl phenols formed on alkylating phenol with propylene trimer, benzene sulfonic acid and KU-2 cation-exchange resin being used as the catalyst. Alkyl phenols of the following structure are obtained on using benzene sulfonic acid: di-ortho-para-, ortho-para-, and para-derivatives. The formation of 2,4,6-trisubstituted screened (sterically hindered) alkyl phenols when a branched hydrocarbon (propylene trimer) is used and the formation of 2,6-disubstituted screened alkyl phenols when normalα-olefins are used [2–4] illustrates the interrelationship between the chemical structure of the alkylating agent and the structure of the alkyl phenols formed and the quantitative ratios of the alkyl derivatives in the reaction product. 2. It has been shown that either 2,4,6-trialkyl phenols or para-alkyl phenols are predominantly formed,depending on the physico-chemical conditions of the alkylation of phenol by propylene trimer in the presence of benzene sulfonic acid: when the molar ratio of phenol to propylene trimer is varied from 1∶6 to 1∶1 the ratio between the trisubstituted alkyl phenols and the p-alkyl phenols varies on an average between 4∶1 and 1∶15. This result confirms the intimate relationship between the molar ratio of the phenol and the olefin hydrocarbon and the quantitative composition of the alkyl phenol structures formed during the reaction [1]. When this particular catalyst is used optimal conditions can be obtained (particularly the ratio of phenol to olefin) which will ensure a maximum yield of an alkyl phenol with a specific chemical structure. 3. It is possible by fractionating the isononyl phenols produced when KU-2 cation-exchange resin is used as the catalyst to obtain a para-alkyl phenols fraction containing up to 90% of the latter