The action of nitric oxide on the β crystalline forms of the metal phthalocyanines of the first transition series from chromium to copper, and on vanadyl phthalocyanine, has been investigated. It has been found that the β modifications of Cr–pc, Mn–pc, and Co–pc (pc = phthalocyanine) react with nitric oxide, as previously reported for the β form of Fe–pc, to form the corresponding mononitrosyl derivatives. In contrast, no reaction of nitric oxide has been observed with the β form of Ni–pc or Cu–pc, or with VO–pc. The reaction of the nitrosyl derivatives of Cr–β–pc and Mn–β–pc with air and with pyridine is also described, and the properties and stability of the various nitrosyl derivatives are discussed in comparison with those of the parent phthalocyanines.