This work investigates the structure and stability of the pyrochlore solid solution PbnNb2O5+n using a combination of high-resolution electron microscopy and electron diffraction. The microwave dielectric properties of the series 1.5 less than or equal to n less than or equal to3.0 have been established, and the effect of the substitution of Ta5+ for Nb5+ on dielectric properties has also been investigated for n = 1.5 and 2.0. Single-phase cubic pyrochlore was obtained for n = 1.5. Compositions for which n > 1.5 showed discreet PbO-rich layers on {111}(cubic) planes between alternating slabs of pyrochlore, similar in appearance to Ruddlesden-Popper phases. The occurrence of these layers results in a lowering of the overall symmetry to trigonal, and their presence was found to have a profound effect on the microwave dielectric properties.