The characteristics of spin system in coal have been compared with and without addition of iodine by means of electron spin resonance spectroscopy. With the addition of iodine. the spectra obtained from low rank coals remain almost unchanged, while those from high rank coals consist of two distinct lines, one narrow and the other broad. The peak intensities of broad component are proportional to the addition of iodine, whereas those of narrow component do not change over the range of iodine doping. Microwave power saturation characteristics of broad component are altered from homogeneous case to inhomogeneous one by iodine doping. (C) 2002 Elsevier Science Ltd. All rights reserved.