Mn2+ ions incorporated into kerogen isolated from the Zvonce black shale (Silur, Eastern Serbia) -were detected by electron spin resonance (ESR) spectroscopy. Similar species were found in dark shale (Silur/Devon, central Morocco). No Mn2+ ions were previously detected by ESR spectroscopy in any geoorganic material. The Mn2+ ions in question were investigated at X- and Q-band frequencies at temperatures of liquid helium (ca. 4.2 K) to 250 K. The spectra exhibit the usual six line pattern with a high g factor of 2.154 +/- 0.005 and an isotropic hyperfine constant (A) of 7.56 +/- 0.1 mT. These six lines are identical but asymmetric at all temperatures and microwave bands (X and Q). A computer simulation revealed that the line asymmetry is probably due to the superposition of the two (or possibly more) very similar Lorentzians for different Mn2" sites. The line width [Delta Hp-p(T)] of the Mn2+ ESR lines shows a strong dependence on temperature with a T-2 variation of the relaxation time. It is suggested that the Raman relaxation process is mainly responsible for this behavior.