Abstract Due to their unique properties, the single walled carbon nanotubes (SWNTs) are very interesting candidates for the development of new electronic devices and sensors. In order to be able to obtain devices with similar and reproducible properties, it is necessary to know the purity of SWNTs obtained di®erently, even from synthesis methods for large scale production. Electron Spin Resonance (ESR), which is a microwave resonant absorption method, can be a useful characterization method, as it is very sensitive to the impurities and defects. Until now, macroscopic amounts of SWNTs can only be obtained as mixtures of tubes with di®erent electronic properties. For this reason the research has always been interested in a simple, fast, and reliable method to detect the features of the SWNTs. In this work, we discuss electron spin resonance parameters on se- veral types of SWNTs and graphite. We recorded the ESR spectra of di®erent SWNTs, obtained by di®erent methods. By comparing room temperature ESR spectra of CVD, arc discharge and laser ablation SWNTs we ¯nd evident di®erences in their parameters. It is important to note that ESR spectroscopy is very sensitive to the presence of residual magnetic catalyst particles, which are usually needed to produce SWNTs. The simulation implemented by the ad- dition of three lines allows the ¯tting to the experimental spectra. For this reason, we also use arc discharge SWNTs synthesized with non-magnetic catalyst. For a further elucidation of the signals ob- tained, spectra at lower temperature have been recorded, and their linewidth and intensity analyzed. The double integration of the ESR line is calculated in order to esta- blish a relationship between this area and the corresponding concen- tration of SWNTs. In order to corroborate our results, we compare the ESR spectra results with their corresponding Raman spectra.