Terahertz time domain spectroscopy (THz TDS) has a wide variety of applications regarding characterization and inspection of dielectric materials such as plastics, ceramics and wood. Market prices for high-performance equipment have decreased below a hundred thousand Euros already and will soon reach levels where a wide range of industrial applications can be envisaged. Besides the detection of flaws and foreign bodies in many types of plastics and measurement of coating thickness, THz TDS can detect diffusion of water into plastics due to the high absorption coefficient of water in the THz range. This contribution regards imaging of water diffusion into a sheet of polyamide 6.6 with scanning THz TDS equipment in transmission mode. The weight increase of the water-exposed part of the sample due to water absorption was about 3% after about 300 hours of immersion, and the exposed area showed a significant decrease of the transmitted THz amplitude. Future work aims at reconstructing the moisture gradient into depth by means of inverse profiling algorithms.