This paper describes the procedure for determining the isotopic ratio of carbon (13C/12C) expressed in delta (δ13C) in the range -46.6 ‰ to +37.63‰ by system composed of an elemental analyzer coupled to a mass spectrometer of isotope ratios (EA-IRMS) in the stable solid isotopes laboratory at the Servicio Geológico Colombiano. The reference materials used in the analysis were IAEA-LSVEC, USGS-43, NBS-19 and USGS-41. The results allow us to conclude that the EA-IRMS is suitable for determination of δ13C in coals as a matrix, in the range evaluated; in a reliable and comparable with any other other laboratories thanks to the acceptance criteria and the statistical analysis performed.