The change in chemical shift of the resonances for both (thio)urea protons and the acridinone N-H proton upon addition of chloride were fitted to various binding models using the Bindfit applet. Fitting to a 1:1 receptor:guest binding model was found to result in a poor fit; however, fitting the data to a 1:2 receptor:guest binding model provided a significant improvement. Fcovfit was calculated for each of the receptors and the covariance of fit of the 1:2 binding model was found to be 8 times greater than the 1:1 binding model for compound 6, and at least 13 times greater for the other receptors. The reversal in direction of the acridinone N-H proton resonance during titrations of the bis-urea species is additional evidence in support of a higher-order binding model. The fit complementarity suggests that two binding events occur over the course of the titration. The K11 and K12 binding constants for receptors 1-10 are displayed in Table .