Building on previous studies of anion-receptor complexes based on a urea scaffold substituted symmetrically with electron-withdrawing nitro groups, the electron density distribution in an analogous thiourea receptor complex and the related asymmetrically substituted urea and thiourea receptors are described. On this basis it is possible to probe both the effect of changing the receptor core from a urea to 10 a thiourea moiety and that of asymmetrical substitution of the receptor molecule. These modifications are shown to significantly alter the anion binding properties, solid-state packing and electron density distribution in the anion-receptor complexes.