Polyaza-receptors 1-3 and pyridine were investigated as complexing agents for phosphoric acid diesters in chloroform. These receptors were used to determine the optimum cavity size for complexing phosphoric acid diesters and to measure the strength of interactions formed by individual host hydrogen bond donors and accepters. Inflections in either the P-31 or H-1 NMR isotherms were found for all receptors. The NMR data indicate equilibria involving host-guest and host-(guest)(2) complexes. Formation of 2:1 guest-to-host complexes is a result of the large dimerization constants for phosphoric acid diesters. The association constant of dinaphthyl hydrogen phosphate to dibenzyl hydrogen phosphate was determined to be 6.5 x 10(4) M(-1). Since strong aggregation of the phosphoric acid diesters complicated the analysis of the NMR data, the binding constants with the polyaza-receptors were determined by a combination of fluorescence and UV/vis techniques. The 1:1 binding constants measured in chloroform for dibenzyl hydrogen phosphate or dinaphthyl hydrogen phosphate with receptors 1-3 and pyridine are 7.8 X 10(3), 8.9 X 10(4), 1.3 X 10(3) and 3.6 X 10(2) M(-1), respectively. The strength of complexation of phosphoric acid diesters to the polyaza-clefts is dependent upon the number of hydrogen bonds formed and the receptor cavity size. The major driving force for complexation arises from the hydrogen bond formed between the phosphoric acid hydrogen and the pyridine nitrogen of a receptor.