Carbon nanotubes (CNTs) are interesting candidates for the development of new electronic devices and sensors due to their small size and their mechanical, optical and electrical properties [1-6]. Well-characterized nanotubes are essential for these applications. Therefore, it is necessary to know the purity of single-walled carbon nanotubes (SWNTs) obtained by different methods, including large scale synthesis. Electron spin resonance (ESR) is a very sensitive probe that can provide information on the chemical environment of unpaired electrons. Raw samples of SWNTs often contain defects and high amounts of paramagnetic ions that are used as catalysts during their synthesis. ESR spectroscopy is sensitive to these impurities and defects [7]. Previous ESR research on CNTs showed that the ESR spectra of purified nanotubes at room temperature consist of up to three overlapping lines that are attributed to the conduction electrons (an asymmetric or Dysonian line), to defects and to the magnetic ions used as catalysts during the synthesis, respectively [8]. Studies of the conduction electrons of SWNTs by ESR have not always been effective. In the SWNTs produced by carbon arc discharge of graphite rods and laser ablation, metals have been added to the carbon target as a catalyst. As a result, the sample contains magnetic catalysts. The magnetic particles used as catalysts during SWNTs synthesis are seen as a very wide resonance line (500-900 G) located at g≈2,1 which prevents observation of the expected conduction electrons line from SWNTs. For this reason, one of our samples was SWNTs synthesized by arc discharge with non-magnetic catalysers. In this paper, we report ESR studies on several carbon nanotubes synthesized by different methods.