​Various laboratory and field techniques used to study geomaterials utilize the principles of electromagnetism. Applications in geotechnical engineering such as resistivity profiling, induced polarization, ground-penetrating radar, and time-domain reflectometry, focus on the electrical properties of the materials and neglect the magnetic component. Yet the a priori assumption that the medium is nonferromagnetic may lead to significant errors in data interpretation because the presence of ferromagnetic materials affects the propagation of electromagnetic waves. An experimental study is conducted using kaolinite with ferromagnetic inclusions. Results show that magnetic permeability is a function of the volume fraction and spatial distribution of ferromagnetic inclusions; the interaction among inclusions increases with increasing volume fraction and proximity. These results and published data show that a relaxation due to ‘‘wall bowing’’ occurs at kilohertz frequencies, and a relaxation due to ‘‘wall displacement’’ occurs at megahertz frequencies. It is shown that the relative permittivity (real and imaginary components) inferred from wave-propagation measurements is larger than the actual value by a factor approximately equal to the real relative magnetic permeability. Corrections for ferromagnetic effects must be computed using parameters measured at the same frequency.