Aryl CH hydrogen bonds (HBs) are now commonly recognized as important factors in a number of fields, including molecular biology, stereoselective catalysis, and anion supramolecular chemistry. As the utility of CH HBs has grown, so to has the need to understand the structure activity relationship for tuning both their strength and selectivity. Although there has been significant computational effort in this area, an experimental study of the substituent effects on CH HBs has not been previously undertaken. Herein we disclose a systematic study of a single CH HB by using traditional urea donors as directing groups in a supramolecular binding cavity. Experimentally determined association constants are examined by a combination of computational (electrostatic potential) and empirical (am and ad values for substituent effects. The dominance of electrostatic parameters, as observed in a computational DFT study, is consistent with current CH HB theory; however, a novel anion dependence of the substituent effects is revealed in solution.