The composition of patina from copper structure of various ages in representative localities has been critically examined with the view to ascertaining whether definite formulæ could be ascribed to the principal constituents. Complete agreement with the formula of the corresponding mineral has been realized in products after 70 years' exposure and upwards. Basic copper sulphate (under most conditions the major constituent) then coincides in composition with brochantite, CuSO₄.3Cu(OH)₂; basic copper chloride (in products near the sea-board) with atacamite, CuCl₂.3Cu(OH)₂; basic copper carbonate (usually present but in minor proportion) with malachite, CuCO₃.Cu(OH)₂. After shorter periods of exposure the basicity of the product is lower than that of the corresponding mineral.