The crystal structure of defect pyrochlores in the PbO - Nb2O5 system have been investigated by powder x-ray diffraction, electron diffraction, and high-resolution electron microscopy. Single-phase cubic pyrochlore was obtained for PbaNb2O5+n (n = 1.5); but for n > 1.5, discreet Pb-rich layers were observed in a manner similar to Ruddlesden Popper and Aurivillius phases. The appearance of these defect layers, which form on <111> planes of the parent pyrochlore, results in a lowering of the overall symmetry to trigonal.