The development of electromagnetic systems to detect and monitor coal‐related processes requires a thorough knowledge of the electrical properties (dielectric constant and conductivity) of coal. Using a two‐path interferometer at microwave frequencies (?9 GHz), samples of solid eastern bituminous and eastern anthracite coals were tested as a function of polarization and direction of travel of the electromagnetic wave. Temperature moisture, and pyrite distribution tests were performed on eastern bituminous coal samples. In general there were slight decreases in the values of the dielectric constant and conductivity as a function of temperature, from ambient to 700° F, and as the moisture content decreased through drying. Pyrite layers along the bedding planes cause a general increase in the conductivity. The anthracites (higher rank coals) have larger values of conductivity (about a factor of 10) and permittivity (about a layer of 2 and 3) than bituminous (lower rank) coals. No distinct relationship was found between the direction of propagation and the electrical properties. Recommendations are made for orienting electromagnetic radiators to improve efficiencies of systems which are designed for coal‐related detection, monitoring and mapping applications.