Monomeric phthalocyanine pentylester derivatives containing Cu and Zn were synthesized via the cyclotetramerization of 4,5-dipentylphthalonitrile. Conversion of the pentylester derivatives by hydrolysis yielded the octacarboxylic acid phthalocyanine derivatives in high purity, without any contamination by oligomers. The dielectric properties of pressed pellets were investigated as a function of frequency. When exposing the phthalocyanine powders to water vapor, a high increase in the dielectric constant was found for the carboxylic acid derivatives, whereas the esters showed no effect. For Cu containing octacarboxylic acid phthalocyanine derivatives, the low-frequency dielectric constant of the pellets was raised from 12 to > 1 x 10(5) when going from a water content of 6.4 to 14.7 wt %, respectively. The concomitant sharp rise in conductivity was attributed to the protons released from the acid groups after water uptake. The present work clearly demonstrates that water uptake and not oxygen is the major factor for achieving surprisingly high dielectric constants in carboxylic acid phthalocyanine derivatives.