Chemisorbed atoms on single‐crystal metal surfaces frequently form commensurate diperiodic structures because of the strong, site‐specific binding. Since the adatoms are well described by the lattice gas model, these systems are valuable testing grounds for recent theories of two‐dimensional critical phenomena. We review briefly the developments in this field, emphasizing our results for the system O/Ni(111). This study includes an analysis of the multicritical low‐coverage phase diagram,Monte Carlo simulations of the overlayer, and the first measurement of critical exponents for a phase transition in a chemisorbed overlayer probed via low energy electron diffraction. We compare our results with predictions of various two‐dimensional models.