ESR experiments from PtRhRe grown and highly semiconductor—metal separated single-walled carbon nanotubes (SWCNTs) were performed at 9.45 GHz and temperatures T between 0.39 and 200 K. No explicit evidence was found for a response from itinerant electrons in the metallic tubes. Rather, in both the metallic (M) and the semiconducting (SC) tubes, the ESR signal originates from quasi-localized defect spins but interactions with free electrons render the two systems characteristically different. The spin susceptibility was of Curie–Weiss type for T ≥ 10 K. For annealed samples it drops for lower T indicating a transition to a ferromagnetic state. Linewidths decrease and increase with increasing T for M and SC tubes, respectively. As a consequence they cross for the two systems. Interaction of spins with free carriers in M tubes via an RKKY type mechanism and increase of linewidth with temperature for SC tubes due to spin–lattice interaction is suggested to be responsible for this.