This paper presents the first results of the TerraDew research project about the effects of free vegetation water on the radar backscatter signal. The investigations are based on intensive SAR and field measurements of diurnal free vegetation water changes in different vegetation types. The results show a systematic backscatter influence in dependence of the investigated backscatter features (wavelength, polarization, vegetation structure, amount of free vegetation water). The highest backscatter increase is found at C-VV and for grassland (up to 4.5 dB), no significant backscatter change is indicated for L-VV and forested areas. The further investigations will focus on the physical description and simulation of these effects to include them in backscatter models and the evaluation of the impacts for thematic SAR data analysis.