An emerging strategy for the removal of solid organic residues in semiconductor device fabrication employs ozone gas delivered to a thin layer of aqueous solution covering the substrate. This process involves the complex reactions of aqueous ozone and of the resulting radical intermediates. Under certain conditions the removal rate has been found to vary with the liquid layer thickness, indicating the importance of mass transport. However, mass-transport limitation of the heterogeneous reaction has never been distinguished from possible competition between diffusion and homogeneous decay of ozone within the boundary layer. We present a simplified kinetic model incorporating both heterogeneous and homogeneous reactions coupled to diffusion. Analytical solutions to this model describe how the rate-limiting step varies with the rate constants, diffusion coefficient, and process parameters. In particular, we show that variations in the liquid layer thickness can drive the process from one mechanistic regime to another. The analysis provides a basis for characterizing and optimizing process conditions. (C) 2008 The Electrochemical Society.