This article is the first part of a two-part study that exemplifies how to treat the solubilization of water in multicomponent surfactant-based systems. In particular, it aims at clarifying the role of cosurfactants in water Solubilization in these systems. The judicious selection of the components in such systems to maximize water solubilization is occasionally thought to be dictated by the chain length compatibility principle, which may be expressed quantitatively by the BSO (Bansal, Shah, O'Connell) equation. Here we demonstrate some limitations of the equation. For example, in our best model system, C-12(EO)(8)/dodecane + pentanol = 1:1 (by weight)/water at 27 +/- 0.2 degrees C, the BSO equation predicts that no alcohol is needed for maximum water solubilization, contrary to our experimental findings. We discuss how to optimize the alcohol/oil weight ratio needed for stabilizing four-component microemulsions. In our model systems C-12(EO)(8) or C-18:1 (EO)(10)/pentanol/dodecane/water, this optimal weight ratio is 1: 1. We also highlight the difference between the effect of normal alcohols on water solubilization-which passes via a maxinnuin-and their effect on percolation processes and structured changes of proteins, which depends solely upon the alcohol hydrophobicity. For the investigation of the effect of branching on phase behavior the utilization of an extended form of the geometrical branching factor F-b is suggested. The meaning of this factor is elucidated by comparing it with topological indices. (c) 2005 Elsevier Inc. All rights reserved.