Within this work unsubstituted rare-earth nickelates have been investigated with respect to fundamental material properties and long-term stability relevant for application as cathodes in solid oxide fuel cells (SOFCs). External parameters such as temperature, oxygen partial pressure and gas phase impurities were adjusted to represent conditions expected in real intermediate temperature (IT) SOFCs. Long-term degradation experiments have been performed for at least 1000 hours to allow for a reliable interpretation and a meaningful extrapolation to longer periods of operation. Although emphasis was put on the application of nickelates as SOFC-cathodes, the results may also be transferred to other applications as for example ceramic membranes used for oxygen separation. As a working hypothesis of this thesis it was assumed that the absence of alkaline earth elements in the investigated materials might be beneficial with respect to long-term stability under realistic IT-SOFC operating conditions. Thus, investigations were limited to La2NiO4+d, Pr2NiO4+d and Nd2NiO4+d, as these compounds do not require partial substitution with alkaline earths or other elements to obtain phase-pure structures. Despite showing very promising results with respect to oxygen exchange kinetics, Pr2NiO4+d was not further investigated in long-term degradation experiments due to its poor phase stability under IT-SOFC operating conditions. The conductivity relaxation method has been proven as a useful method for determining oxygen transport parameters as well as for monitoring degradation phenomena occurring on the sample surface in various atmospheres. Two sample geometries with different contacting schemes have been employed in relaxation measurements, where appropriate sample and contact designs are necessary to ensure specific diffusion patterns within the sample required for a correct modeling of the oxygen exchange process. Particular caution was necessary when transferring models developed for weight relaxation measurements to conductivity relaxation experiments. Metal-coatings were sputtered on samples of La2NiO4+d and Nd2NiO4+d to obtain reliable values for chemical diffusion coefficients. The conductivity relaxation method was also successfully employed for the investigation of long-term degradation phenomena. In this case, diffusion coefficients are assumed to be of less importance for characterizing degradation processes, which are expected to affect primarily the immediate sample surface.