The relative stability of 10-hydroxy-10,9-boroxophenathrene and its electrophilic reactivity are traced through experiment and calculations to the partially aromatic character of the boron-containing heterocyclic ring. The electrophilic reactivity of 10-hydroxy-10,9-boroxophenathrene towards itself in the solid state and towards other nucleophiles in solution is described. The mechanism of the solid-state reaction has been characterised by both X-ray diffraction and thermal analysis. In solution, however, 10-hydroxy-10,9-boroxophenathrene does not react with itself, although it does react rapidly and reversibly with benzylic alcohols in solution, even at low temperatures. This selective and reversible reaction is ideal for use in self-assembly processes.