The stratification during the simultaneous flow of two immiscible liquids through a horizontal conduit has been investigated. For the identification of different flow patterns, an indigenously developed unique optical probe has been used. The presence of different phase contents and various interfacial features along the optical path gives rise to attenuation and scattering and makes the identification possible. The probability density function (PDF) analysis and the wavelet multiresolution technique have been adopted for development of an objective flow-pattern indicator. The range of existence of the different stratified distributions as predicted by the indicator is in close agreement with the maps reported in literature. (c) 2007 Elsevier Ltd. All rights reserved.