Interfacial contamination of batches of refined petroleum products transported through multiproduct pipelines is a long standing problem in the petroleum industry. Determination of the extent of mixing depends on various factors including fluid properties, operating conditions and flow regimes. The convective-diffusion model along with a fictional parameter, known as the axial dispersion coefficient introduced by Sir G. I. Taylor, is the commonly employed methodology to evaluate longitudinal mixing. However, the model was formulated with a uniform turbulent or laminar flow across the entire flow field. It is argued that the assumption of a uniform flow regime leads to inaccuracies in predicting the extent of contamination at the interface. This study incorporates both the turbulent and the viscous effects in predicting the interfacial contamination volume. Taylor's analysis of dispersion coefficients has been reviewed, and an improved model combining the weighted effects of the viscous boundary layer contributions and the turbulent core contributions to axial dispersion is proposed. The proposed model provides a possible explanation for the underlying physics of interfacial contamination in flow through straight pipes without pipe fittings and other devices. A comparison of the model predictions with literature data demonstrates the importance of considering both turbulent and viscous effects to achieve better overall prediction accuracy. (C) 2012 Elsevier Ltd. All rights reserved.