Dehydrohalogenation and subsequent nucleophilic substitution–addition reactons of pentachlorocyclopentadiene dimer are described. Mechanistic pathways (including the possibility of 2p–4π interaction leading to a stabilised intermediate carbanion) to account for the ready formation of (largely) a single triether when the dimer is treated with potassium methoxide, are briefly discussed. Intramolecular cyclisation and rearrangements of derivatives of the triether (and related compounds) are described.