The instable modifications αI and α II (previously called γ) of copper phthalocyanine are isomorphoua with the stable modifications α I of 4-monochloro-copper phthalocyanine, and α of platinum phthalocyanine respectively. The latter two can be obtained as single crystals and we have determined the essential structural features of them.By isomorphy considerations it was possible to recognize the structural differences between the β, α I and a II modifications of copper phthalocyanine. These α-structures are very similar to those of the αΠ modification of 4-monochloro phthalocyanine and of the α modification of 4-tetrachloro phthalocyanine.The essential difference between the α structures and the well-known β structure of copper phthalocyanine is a different type of one-dimensional molecular stacking. There are several ways of assembling the stacks into a space lattice so that different α structures are yielded.The substitution of peripheral hydrogen by chlorine (in the 4 position) increases the tendency to form α-type stacks and their stabilization.