Islands of monolayer vacancies can be generated on transition metal surfaces by ion bombardment of samples held at elevated temperature. Our experiments show that the average size of the monolayer vacancy islands can be varied in a controlled manner from 3 to 30 nm by adjusting the sample temperature. We also demonstrate that monolayer vacancy islands can be used as two-dimensional nanometer-scale catalytic "reaction vessels." The reactants are confined to the reaction vessels by energetic barriers to adsorbate diffusion across the atomic steps that form the walls of the vessels. Reaction vessels with diameters of similar to3 nm can accommodate only a small number of reactant molecules (e.g., a maximum of 28 ethylene molecules can be adsorbed in an area with a diameter of 3.3 nm on a Pt(lll) surface). Reactions whose products or rates depend on the number of reactant molecules that are available can be controlled using these reaction vessels. This is demonstrated for the particular example of carbon particles that are formed from the dehydrogenation of mono-olefins on Pt.