Field effect transistors (FETs) have been fabricated with metal phthalocyanines and rare earth diphthalocyanines. The influence of the metallic ion in mono‐ and diphthalocyanines and the conditions of FET fabrication on electrical characteristics has been determined for devices tested in ambient atmosphere. Aging under various conditions has identified the role of oxygen on the device behavior. Unlike conventional inorganic FETs, these diphthalocyanine based devices work through the modulation of an accumulation layer formed by majority carriers.