The attack of HCl vapors on copper follows this pattern: a flat glossy film of cuprous oxide forms; after some time, the surface becomes rough, due to the formation of a layer of cuprous chloride on top of the oxide. The underlying cuprous oxide layer increases in thickness, and the specimens show the red color characteristic of this compound. Later the surface of the specimen becomes darker and can become almost black, due to formation of a thick film of CuO on the surfaces of the CuCl crystals. Finally basic chloride also forms; it takes two different growth patterns. Mostly it forms as outgrowth at isolated points; in addition, it occurs as a thin layer on the surfaces between the points of attack. The writer notes the general statement that the basic copper chloride formed by corrosion is identical with natural atacamite. However, he found that basic copper chloride occurred in four different polymorphic forms. The species formed in his corrosion studies was not identical with atacamite, but rather with the form designated as the gamma-modification, and corresponded to paratacamite. At higher HCl concentrations the normal chloride formed (CuV+Cl₂.2H₂O). 1.5-4 Cu are destroyed per one HCl. The outgrowths of basic copper chloride were found in every case to occur over a pit in the metal that was partly filled with cuprous chloride. From the appearance and distribution of the outgrowths it appeared that they formed where tiny droplets of moisture condensed. The relationship between solubility and pH of solutions for the various copper corrosion products are shown graphically as follows: (graph). The equilibrium constants show that at anodic sites only Cu₂O can precipitate; the concentration of Cu^-- </sup> is too low to exceed the solubility product for CuO. Up to a pH of 2, Cu₂O is the stable phase; in more acidic HCl solutions, CuCl can form. Thus, as the Cu₂O layer increases in thickness, diffusion of the HCL into the interior is reduced, and the HCl concentration on the oxide surface can rise high enough to cause CuCl to begin to form. - AATA