Electron spin resonance investigations on single wall carbon nanotubes dispersed in polyethylene are reported. Three resonance lines were observed; a wide line assigned to magnetic iron clusters (catalyst residues), a broad and intense line originating from uncoupled electrons delocalized over the conducting domains of carbon nanotubes (in interaction with the electronic spins assigned to magnetic impurities), and a faint line superimposed on the broad one, assigned to paramagnetic impurities. The temperature dependence of resonance line parameters (resonance line position, width, and double integral) in the range 150-450 K has been analyzed. It was observed that the parameters of the broad and narrow lines are sensitive to the glass and melting relaxations occurring within the polymeric matrix.