Electronic properties of multi-walled carbon nanotubes are investigated with electron spin resonance (ESR) spectroscopy in light of their application as chemsensors. The nanotubes were exposed to ammonia both before and after an oxidative purification procedure. Ammonia exposure affects most strongly the resonance stemming from conduction electrons at ambient temperature, and that from intrinsic defects at 5K. The strong interaction with ammonia even at ambient temperature holds promise for use of multi-walled carbon nanotubes as chemsensors, as also shown in other work. The ESR technique provides a simple method to investigate the potential of a sensor material in bulk. (c) 2007 Elsevier B.V. All rights reserved.