The polymorphism of lithium phthalocyanine (PcLi) was studied for powders as well as for thin films deposited on glass substrate. Powders were yielded by two electrochemical synthesis routes. While the powders obtained by electrochemical oxidation of PcLi, in acetonitrile and acetone lead to an x structure, the synthesis route proposed by M.A. Petit leads apparently to the ct form of PcLi. Their very different sensitivity to oxygen, observed using electron spin resonance (ESR) spectroscopy, is explained by means of the crystallographic structure of both polymorphs. Thin films were prepared by vacuum deposition and phase transitions as well as preferential ordering of the crystallites are studied as a function of increasing substrate temperature. While films deposited at low substrate temperatures, below 150 degrees C, lead to an x-like form with the molecular columns lying in the plane of the substrate, the higher substrate temperatures favour the growth of the a form and a loss in preferential ordering. Accordingly, changes in the sensitivity to oxygen are followed as a function of substrate temperature.