The second-order Raman band (omega(G')) for single wall carbon nanotubes (SWNTs) in the range similar to 2600-2700 cm(-1) depends on the excitation energy and the diameter. However, the diameter dependence is not well established and understood. Here we propose that the two different physical dependencies are separable and can be taken into account by a phenomenological here derived equation. Using this methodology we have been able to find consistency between data for double-WNTs and large diameter SWNTs. Further, the determined diameter dependence of G' correctly predicts the frequency for an infinitely large tube diameter, which is equivalent to a planar graphene sheet. (c) 2006 Elsevier B.V. All rights reserved.