The position and intensity of the pi plasmon absorbance of a series of single-walled and multi-walled carbon nanotubes has been systematically investigated for the first time using ultraviolet-visible (UV-vis) absorption spectroscopy. All nanotube samples obey the Lambert-Beer law and exhibit similar extinction coefficients (similar to 50 mL/mg cm) regardless of their structure or method of preparation. The absolute wave-length (energy) of the pi plasmon absorbance has been shown to vary with nanotube diameter as E-pi = 4.80 + 0.70/(d(NT))(2). This empirical relationship when correlated with nanotube pyramidalisation angle can be used to estimate the energy range for the pi plasmon in a single sheet of graphene to be 4.8-4.9 eV. (C) 2010 Elsevier B.V. All rights reserved.