Temperature and pressure dependent electron spin resonance (ESR) studies were conducted on two oxidatively purified single-walled carbon nanotube samples. For both samples, the ESR signal is composed of two components. The narrower component is ascribed to localized spins due to defects, the broader one to conduction electrons. Hydrogen predominantly interacts with defects. Below the condensation point, addition of hydrogen is suggested to cause diamagnetic dilution of the conducting sample, which leads to increased signal intensity. (c) 2008 Elsevier B.V. All rights reserved.