Raman spectroscopy in conjunction with quantum chemistry allows efficient inspection of the electronic and structural properties of biradicals. Investigations on a series of quinoidal oligothiophenes uncover a thermal equilibrium between singlet and triplet biradical states (see picture) for the longest of the investigated systems. Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2002/2007/z703244_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.