There has been growing interest in the modeling of proton exchange membrane fuel cells (PEMFC) over the last two decades. While a variety of steady-state models have been proposed, literature is scarce in PEMFC dynamic models and transient studies. Typical dynamic models for PEM fuel cell are empirical current-voltage relationships. The internal transients associated with reactant and product species and other components are usually neglected. A detailed dynamic model for spherical agglomerate in PEM fuel cell is presented in this work. The dynamic model includes detailed mathematical equations for conservation of oxygen and hydrogen ions inside the agglomerate. The agglomerate dynamic model is simulated for typical operating conditions inside the PEMFC catalyst layer. Simulation studies show that the time scales in which the dynamics of agglomerate potential and concentration of dissolved oxygen respond differ by several orders of magnitude. Transient response of agglomerate current to step changes in surface boundary conditions are also presented. Reasons for the typical characteristics observed in the dynamic behavior of agglomerate current are also highlighted. (c) 2005 Published by Elsevier B.V.