Abstract We compare the performance of organic transistors based on sodium salts of sulfonated metal phthalocyanines (MePCS x s) to those based on their non-sulfonated counterparts (MePCs). The cases of Me Co, Al are discussed. MePCS x s are water soluble, and therefore can be applied by spin coating, compared to MePCs which require vacuum evaporation. Furthermore they present much higher field mobilities and require much lower operating voltages ( V SD < 1 V and V G < 10 V). This feature is due to the presence of the Na + counter ions, a situation similar to organic transistors with electrolyte dielectrics. The current–voltage characteristics depend strongly upon the degree of sulfonation, which has to be controlled in order to optimise device performance.