Pentafluorophenol reacts with sodium trichloroacetate in monoglyme in a Reimer-Tiemann reaction to give not only pentafluorophenyl orthoformate (6) but also a previously unknown 'abnormal' type of product, tetrakis(pentafluorophenoxy) ethene (5), one possible formation of which could be by dimerisation of an intermediate carbene. (C) 1998 Elsevier Science S.A. All rights reserved.