An accurate determination of the hydrophilic-lipophilic nature of surfactants plays an essential role in guiding the formulation Of microemulsion with the goal of achieving low interfacial tension (IFT) and high solubilization. While several empirical models have been proposed as simple tools for predicting surfactant characteristics and microemulsion conditions, only a few of these models are fundamentally based yet convenient to use. In this work, the hydrophilic-lipophilic deviation (HLD) approach was used with mixed surfactant systems to determine the surfactant characteristic (sigma) and the sigma/K parameter of conventional and extended surfactants. To our knowledge, this is the first time that the HLD index has been used to represent the hydrophilic-lipophilic behavior of extended surfactants. it was observed that inserting PO and/or EO groups in extended surfactants play a key role in altering sigma values and sigma/K Values. Finally, the sigma parameters found in this work were combined with the HLD equation and used to demonstrate its practical utility for guiding the optimum formulation (in this case, optimum salinity) for a microemulsion system. (C) 2008 Elsevier Inc. All rights reserved.