The ionic conductivity of ceramic GdxZr1-xO2-x/2 and NdxZr1-xO2-x/2 solid solutions is measured as a function of composition. Both solid solutions are of the defect fluorite type and the fluorite-related ordered pyrochlore phase is present around the stoichiometric compositions Gd2Zr2O7 and Nd2Zr2O7. As a function of composition the conductivity goes through a maximum for stoichiometric Gd2Zr2O7 whereas the conductivity is minimal for Nd2Zr2O7. It appears that at the stoichiometric pyrochlore composition both, activation enthalpy H and pre-exponential factor 0 are minimal and that the absolute magnitude of the pre-exponential factor accounts for the difference in the net conductivity effect of the systems studied. The results obtained from both, single crystalline and ceramic samples can be correlated with the defect structure and ordering in the anion sublattice and with deviations of Vegard's law in the fluorite subcell dimensions. No discontinuity of Q0 or δH is observed at the fluorite-pyrochlore phase boundary. This suggests the existence of a hybrid phase around the fluorite-pyrochlore phase boundary in this type of system.