The relationship between surface crowding and catalytic activity was investigated using thiolate self-assembled monolayers (SAMs) on Pd/Al2O3 catalysts. The surface density of the thiolate modifier was controlled by varying the steric bulk of the organic substituent. A straight-chain alkanethiol 1-octadecanethiol (C18), with a nearest-neighbor spacing of similar to 4.7 angstrom on Pd(111) surfaces, created a denser SAM coating than 1-adamantanethiol (AT) with a nearest-neighbor spacing of similar to 6.4 angstrom. Diffuse reflectance infrared spectroscopy revealed that CO adsorbate molecules adsorbed only on threefold hollow and atop sites on C18-modified surfaces. On AT-modified surfaces, however, access to bridging and additional linear sites was also observed. Analysis of adsorption isotherms suggested that CO adsorption energies were comparable on AT-modified and C18-modified catalysts. Acetylene hydrogenation, which results in uncontrolled crowding due to carbonaceous "coke" formation on the catalyst, was found to be insensitive to modification by the thiols. For hydrogenation reactions less associated with uncontrolled coking, crowding - and therefore reactivity - could be controlled systematically using SAMs. In particular, ethylene hydrogenation was 17 times faster on AT-coated surfaces than on C18-coated surfaces, consistent with the additional accessibility to specific sites unavailable on C18-modified surfaces. The effect of modifier density on reactivity was found to be dramatically different for several mono- and bi-functional reactants in a manner consistent with previous literature reports, suggesting that controlled crowding with SAMs can be used to understand reaction structure sensitivity and active site requirements in catalysis. (c) 2013 Elsevier Inc. All rights reserved.