The structural change of diamond induced by hydrogen plasma exposure at room temperature, and its thermal stability, were investigated using electron spin resonance (ESR), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) techniques. ESR observation revealed that it gave rise to a highly defective structure (spin concentration of similar to 10(20) cm(-3)) which is very similar to the structure of hydrogenated amorphous carbon (a-C: H) confirmed by XPS and FTIR. Post-annealing was also carried out to clarify the thermal stability of the defects. The number of spin centers decreased with increasing annealing temperature, and eventually, the defective structure changed to a graphitic one by annealing at 800 degrees C. (c) 2005 Elsevier B.V. All rights reserved.