Studies of electron spin resonance (esr) are reported on several types of single-walled carbon nanotubes (swcnts) synthesized by different methods. We work both with the obtained samples, which contained randomly distributed semiconducting and metallic swcnts, and with selected 99% semiconducting or metallic swcnts. Our aim is to use esr spectroscopy in order to assess the quality of carbon nanotubes. We analyze the temperature dependence of esr spectra in the range 4–300?k and report the temperature dependence of the linewidth, the intensity and the asymmetry factor (i+/i-) of the esr lines. The esr response from the arc discharge swcnts produced using non-magnetic pt/rh catalyst and from the selected semiconducting tubes exhibits an asymmetric line with an intensity that remains almost constant with decreasing temperature, showing a constant linewidth of about 2.9?mt. For selected bucky paper (bp) metallic or semiconducting carbon nanotubes, we also study the esr signal according to the bp parallel or perpendicular orientation to the magnetic field. The g-factor and the asymmetry factor show a clear anisotropy.