A new polyamine macrobicyclic compound was synthesised through a [1+1] "tripod-tripod coupling" strategy and using a Schiff base condensation reaction, followed by sodium borohydride reduction. The resulting compound is a heteroditopic cage (btpN(7)) in which one of the head units is appropriate for the coordination of copper(II), whereas the other head is available for additional hydrogen-bonding and electrostatic interactions with substrates. The acid-base behaviour of the new compound, the stability constants of its complex with the Cu(2+) ion and the association constants of the copper( II) cryptate with oxalate (oxa(2-)), malonate (mal(2-)), succinate (suc(2-)), maleate (male(2-)) and fumarate (fum(2-)) were determined by potentiometry at 298.2 K in aqueous solution and at an ionic strength of 0.10 mol dm(-3) in KNO(3). These studies revealed a clear preference of the receptor [CuH(h)btpN(7)H(2)O]((2+h)+) for oxa(2-) over the other dicarboxylate substrates. This arises from co-operativity between metal-anion coordination and electrostatic and hydrogen-bonding interactions, in accordance with the ideal size of this dicarboxylate, which allow it to take full advantage of the potential binding sites of the receptor. A qualitative indicator-displacement study, in agreement with the potentiometric studies, demonstrated that the copper cryptate receptor can be used as a selective visual sensor for oxalate.