We have studied the effect of niobium doping on the electrical conductivity of Yb2Ti2O7-based oxygen ion conductors. Yb-2[Ti1-xNbx](2)O-7 (x = 0.01, 0.04, 0.1) and (Yb0.8Tb0.1Ca0.1)2[Ti1-xNbx](2)O-6.9 (x = 0; 0,05; 0.1) pyrochlore solid solutions were synthesized through coprecipitation followed by firing at 1550 degrees C for 4 h. The materials were examined by XPS, XRD, scanning electron microscopy and impedance spectroscopy. Yb2(Ti0.99Nbos1)207 was shown to have the highest oxygen ion conductivity in air (23 x 10(-3)S/cm at 750 degrees C), which is however markedly lower than that of undoped Yb2Ti2O7. In the (Yb0.8Tb0.1Ca0.1)2[Ti1-xNbx](2)O-6.9 (x = 0; 0.05; 0.1) system, the highest conductivity is offered by(Yb0.8Tb0.1Ca0.1)(2)[Ti1-xNbx](2)O-6.9 (sigma= 4.44 x 10(-3) S/cm at 650 degrees C). Additional oxygen vacancies created by Ca doping in pyrochlore structure reduce the detrimental effect of Nb4+ doping on the oxide ion transport up to 5% Nb. The conductivity of the Yb-2(Ti(0.99)Nb(0.0)1)(2)O-7 and (Yb0.8Tb0.1Ca0.1)(2) [Ti0.95Nb0.05](2)O-6.9 solid solutions was measured both in air and under reducing conditions (5% H-2 in N-2 and CO2 atmospheres). A comparative study of both these compositions under 5% H-2 in N-2 atmosphere showed that the transport mechanism was not affected by complex doping of the lanthanide and titanium sublattices in the Yb2Ti2O7-based materials and was related to oxygen vacancies. Conductivity measurements in CO2 were done to ensure correct evaluation of the ionic conductivity of (Yb0.8Tb0.1Ca0.1)(2)[Ti0.95Nb0.05](2)O-6.9, because in air it seems to be a mixed p-type and ionic conductor. (C) 2014 Elsevier B.V. All rights reserved.