The synthesis and structural characterization of the sterically congested pyrophosphite 6-[2,4,8,10-tetrakis(1,1-dimethylethyl)-dibenzo[d,f][1,3,2] dioxaphosphepin, 3, is described. hi solution at room temperature, a single species was observed that was consistent with a pyrophosphite structure without any evidence for the tautomeric diphosphine monoxide. Below the coalescence temperature (T-C), 0 degreesC, three atropisomers were observed with relative absolute configurations of (R*,R*,R*), (R*,S*,R*). and (R*,R*,S*). Ring inversion of the seven-membered rings below the we is slow on the NMR time scale, which leads to observable diastereoisomerism because of the presence of two independent stereoaxes (sp(2)-sp(2) C-C single, bond connecting the two aryl rings). Additionally, a rotation about an exocyclic P-O single bond connecting the two seven-membered rings, which constitutes a third stereoaxis, is slowed on the NMR time scale. In the X-ray crystal structure of 3, the solid-state conformation was found to be the same as the major conformation in solution below the T-C, namely, the (R*,R*,S*) atropisomer. The results of a conformational search, performed with a specifically parametrized AMBER* force field, were in agreement with the P-31 NMR assignment of the major (R*,R*,S*) atropisomer, which was found to be an energy minimum. Additionally, we could independently assign the relative configuration of the minor isomers based on the calculated results.