One of the soil properties influencing the sensing depth of ground-penetrating radar (GPR) is intrinsic attenuation. Especially in high-frequency investigations, as for explosive ordnance and landmine detection, it plays a major role. In many cases attenuation is the limiting factor that determines the applicability of the GPR system. In order to investigate the frequency-dependence of electromagnetic soil properties, different soil samples from Germany and Afghanistan were analyzed. For this purpose, we used a coaxial transmission line together with a vector network analyzer in the laboratory. Two coaxial line cells were used in order to determine the complex dielectric permittivity in the 1 MHz to 10 GHz frequency range. The complex permittivity curves were fitted by a generalized model, which accounts for different dielectric relaxation mechanisms. The inversion of the model was carried out by means of the Geophysical Inversion and Modelling Library (GIMLi). Splitting the measured complex dielectric permittivity data using the generalized model made it possible to investigate the different energy loss contributions to the intrinsic attenuation, which are in turn attributed to certain soil components. Based on the laboratory results, we intend to create a simple method for the prediction of the GPR performance in the field.