Some layers of thin film photovoltaic modules maybe critically sensitive to moisture. In this study we present a new tool for monitoring the effect of moisture using a particular Transparent Conductive Oxide (TCO) as a sensor. The moisture content of the encapsulant was determined by Fourier Transform Infra Red (FTIR) spectroscopic measurements. The TCO resistivity was measured using an inductive method. The different spectroscopic results show that the diffusion of water vapour in the encapsulant used in this study is in good agreement with Fick’s law and correlate well with the increase of resistivity of the TCO. However, the transport measurements bring evidence for a degradation of the TCO resistivity undetectable by conventional FTIR measurements.