This study concentrates on the external and internal mass transfer with multiple reactions in the catalytic layer of a three-way catalyst (TWC). A single channel model accounting for the species diffusion inside the washcoat using the effectiveness factor was developed. Validation and calibration of the model was achieved by comparing predictions against experimental data obtained previously by the same authors. The model was then applied to study the importance of both turbulent monolith Structures and controlled washcoat structures on TWC conversions. The numerical results show that: (i) increasing the transport coefficients using turbulent monolith structures can produce either positive or negative effects oil the TWC conversions; (ii) overall, the net effect of increasing the transport coefficients on the TWC conversions is positive; (iii) at high inlet gas temperatures and high space velocities the turbulent monolith structures present important improvements in the TWC conversions: (iv) the TWC conversions can be significantly improved enhancing the transport properties of the porous washcoat structure: (v) enhancements in the transport properties of the washcoat structure have deeper impacts oil the TWC conversions than improvements in the monolith channel structure. (C) 2008 Elsevier B.V. All rights reserved.