Chlorohydrocarbons chemisorb dissociatively on Pd surfaces at greater than or equal to 200 K and greater than or equal to 10(-8) Torr, as shown by high-resolution electron energy loss spectroscopy (HREELS) and C Is, Cl 2p, and Pd 3d surface core level shifts (SCLS). From CH2Cl2-generated overlayers on Pd(100), hydrogen is removed thermally and carbon is removed by oxidation as CO and CO2, leaving voids between the chlorine (Cl) ensembles that are accessible to other adsorbates. The resulting Cl overlayers are partially ordered depending on initial conditions. The concomitant low-energy electron diffraction (LEED) patterns show that the Cl ensembles are stable to high-temperature reaction cycles. The order-disorder phenomena observed in the temperature range 300-900 K include the generation of domains consisting of Cl only that surround reactive sites of the metal. With CH2Cl2/Pd(100). these domains are formed by lateral packing of 16 Pd/CCl2 units that restrict the supply of O-(a) for oxidation of C. Selectivity is switched from CO2 to CO with increasing Cl concentration. Lateral interactions are of two types : mobile O-immobile Cl and mobile O-mobile O. This is reflected in a lowering of the O-2 temperature programmed desorption (TPD) maxima with increasing Cl concentration. A statistical-mechanical model is presented for the effects of Cl-(a) with phase-equilibration between a dense and a rare phase of O-(a).