The effect of pyrochlore order in solid solutions with the defect fluorite structure was studied. The amount of order was changed by various heat treatments or by varying the cation composition. Examples of short range order and microdomain-formation have been observed in Tb2Zr2O7+x and Gd2Zr2O7. Pyrochlore order results in a decrease of the activation enthalpy and pre-exponential factor for oxygen ion conduction. The results are explained by a model based on the occurrence of a preferential diffusion path with a low strain energy component in ΔH. Optimal conductivity is achieved in materials which are not completely ordered. Mixed conductivity in Ln2Zr2O7 with P structure can be realized by partial replacement of Ln by Tb. This results in good ionic as well as electronic conductivities.