A theoretical analysis of anthocyanidins, a class of natural plant pigments, has been conducted using density functional theory at the B3LYP/6-31G(d) level. It is found that these compounds are non-planar, with the 2-phenyl ring twisted relative to the benzopyrylium. rings. TD-DFT calculations reveal a first excited state dominated by two orbital transitions, including the HOMO-LUMO transition. From a comparison of these molecular orbitals with those of related compounds, and from the known chemistry of these compounds, it is suggested that anthocyanidins should be regarded as natural, stable carbocations. (c) 2005 Elsevier B.V. All rights reserved.