Water management is crucial to the performance of PEM fuel cells. Water is generated as part of the electrochemical reaction, and is removed through the reactant channels. This results in two-phase flow in the reactant channels. Increased understanding of the behavior of the liquid water in the channels allows us to devise better strategies for managing the water content inside the fuel cell. Most previous work has been focused on qualitative information regarding flow pattern maps. The current work presents new algorithms developed in MATLAB (R) to quantify the liquid water and to identify the flow patterns in the cathode side reactant channels. Parallel channels with dimensions matching those of commercial stacks have been used in this study. The liquid water present in the reactant channels is quantified for different temperature, inlet RH and current density conditions, and the results are presented in terms of area coverage ratio. The dominant flow patterns for the different conditions have been mapped, and trends interpreted on the basis of air flow velocities and saturation conditions within the channels. (C) 2013 Elsevier B.V. All rights reserved.