The effects of n-pentanol and n-octanol on the interfacial tensions of phenylalkane sulfonate monoisomer systems against n-alkanes were investigated for elucidating the mechanism of ultra-low interfacial tensions related to enhanced oil recovery. The addition of n-pentanol to phenylalkane sulfonate monoisomer systems had little effect on the interfacial tensions of 1-phenylalkane sulfonate systems, whereas n-octanol could lower their interfacial tensions up to 1-2 orders for some n-alkanes. n-Pentanol could lower the interfacial tensions of 3-phenyltetradecane sulfonate against n-hexane, n-heptane and n-octane, but n-octanol had little effect on the interfacial tensions of 3-phenyltetradecane sulfonate. The interfacial tensions of 5-phenyltetradecane sulfonate could be lowered by the addition of n-pentanol, but increased upon the addition of n-octanol. The effects of alcohol concentration and time on interfacial tensions were also researched to show that interfacial tensions of 5-phenyltetradecane sulfonate solution increased with time at whether lower (0.04%, vol%) or higher (0.4%, vol%) n-octanol concentrations. (C) 2004 Elsevier Ltd. All rights reserved.