A mesoscopic description of surface chemical reactions, aimed to provide a Link between microscopic lattice models and reaction-diffusion equations, is formulated. Such a description is needed when large populations of nanoscale structures are considered or patterns characterized by a combination of macroscopic and microscopic lengths are investigated. By using the example of an adsorbate with attractive lateral interactions between molecules, the mesoscopic evolution equation for fluctuating coverages is derived from the microscopic master equation of the respective kinetic lattice model. This stochastic equation is applied to study phenomena of pattern formation related to adsorbate phase transitions.