Two major characteristics of monolithic catalysts, which are often ignored in performance considerations, are the cell density and the thickness of the catalytic layer. The impact of these two parameters becomes apparent from a monolithic reactor simulation applied to the selective hydrogenation of phenylacetylene to styrene. The reactor model takes kinetics, diffusion, and mass transfer into account. As functions of the cell density and coating thickness, the required reactor volume for a phenylacetylene conversion level of at least 92% and the yield of undesired ethylbenzene were calculated. With increasing coating thickness, the conversion of phenylacetylene becomes limited no longer by the amount of catalyst present in the reactor but by diffusion. External mass transfer was never found to be rate-limiting. The required reactor volume shows a minimum at the transition from kinetically limited control to internal diffusion-limited control. A higher cell density (smaller channels) favors a smaller reactor. To minimize the conversion of styrene, a very thin coating layer suffices, but this maximizes the required reactor volume.