This work investigates the structure and stability of the pyrochlore solid solution PbnNb2O5+n using a combination of highresolution electron microscopy and electron diffraction. Single-phase cubic pyrochlore has been obtained for a Pb1.5Nb2O6.5 composition. As expected, compositions containing higher [Pb]/[Nb] ratios showed discreet PbO-rich layers between alternating layers of Pb1.5Nb2O6.5, similar in appearance to Ruddlesden–Popper phases. The occurrence of these layers, which lie along <111>c planes, results in a lowering of the overall symmetry to trigonal. These layers and their spacing have a profound effect on the dielectric properties of the ceramics, with a spacing of about 18.3 Å resulting in the most temperature-stable structure.