The time domain reflectometry technique for measuring the volumetric soil water content is highly sensitive to installation effects because the sampling volume is more heavily weighted close to the transmission line elements. To investigate changes in soil structure caused by the installation and its influence on the measurement, field and laboratory experiments covering different probe dimensions, soil types, and installation techniques were performed. The results indicate that merely pushing the probes into the soil entails a significant reduction of the measured water content up to 0.10 cm3/cm3. The effect is strongest close to saturation since the large pores are affected most. The degree of compression differed strongly according to the varying compressibility of the investigated soils. It was smaller for the smaller probe indicating a more favorable relationship between sampling volume and compression. The compression effect could be minimized using a drill to remove the soil for the probes. X ray-computed tomography was used to visualize the installation effects and to measure the density distribution around the probes.¿ 1997 American Geophysical Union