The temperature dependence of d.c. current in organic semiconducting thin films of copper, zinc and nickel phthalocyanines has been investigated in an attempt to observe a phase transformation from the metastable alpha-form to the stable beta-form, as previously observed only in pellets of metal-free phthalocyanine. The films were deposited in the sandwich configuration with ohmic gold electrodes and measurements were made over the temperature range 20-400 degrees C. The influence of breaking vacuum during fabrication of the structures was also studied. For increasing temperatures the copper and zinc phthalocyanines showed a maximum in current at approximately 100 degrees C due to the desorption of water vapour in the alpha-form, whereas a permanent increase in conductivity above approximately 300 degrees C was ascribed to a phase change to the beta-form. Observed discontinuous changes in the current below 300 degrees C were associated primarily with the effects of organic impurities. For decreasing temperatures the films showed well-defined activation energies over a wide temperature range, confirming the existence of the thermally stable beta-form. The results are in good agreement with earlier X-ray diffraction data on cobalt phthalocyanine films.