The resonant second-order G'-band Raman spectra of heterogeneous single walled carbon nanotube (SWNT) dispersions exhibit a bimodal distribution for significantly wide diameter distributions and when coupling between SWNTs is negligible. This is shown to be a manifestation of the simultaneous fulfilment of the double resonance criteria by the incident and the scattered light. (c) 2007 Elsevier B.V. All rights reserved.