The molecular water concentration inside zinc phthalocyanine (ZnPc) thin films was measured. After exposure to air, gas effusion experiments show that the ZnPc layers contain (1.7 +/-0.4) x 10(20) water molecules per cm(3), which corresponds to 1 H2O per 10 ZnPc units. We can distinguish a mobile and an immobilized population of H2O in ZnPc films. The mobile part effuses out at room temperature when exposing the films to a low pressure of 10(-2) mbar, whereas temperature activation is needed to reach a complete out-diffusion of water. The effusion process was observed to proceed with a diffusion coefficient D-H2O of (1.3 +/- 0.3) x 10(-10) cm(2) s(-1) at 296 K. The rate of water effusion directly correlates with the timescale of the decrease of surface conductivity when exposing the layers to an equally low pressure. This indicates the existence of an electrically active surface layer of water molecules, which is refilled from the bulk. of water molecules during the effusion process. (C) 2000 Elsevier Science B.V. All rights reserved.