We have used STM to observe directly a process which can be described as tyro-dimensional, surface-mediated Ostwald ripening, occurring during the decomposition of methoxy intermediates on a Cu(110) surface. Some islands get bigger during the course of the reaction, at the expense of smaller ones, and it is shown that this diffusion is important for dictating the rate of methanol decomposition. This is proposed to occur from the dilute phase of methoxy existing in the inter-island region and the methoxy maintains a pseudo steady state there, supplied by diffusion from, and between, islands of adsorbate on the surface.