The influence of water vapor pressure, P(H2O2), on chemical stability of SrCe0.95Yb0.05O3-x under hydrogen atmosphere was investigated using XRD and Raman spectroscopy. From an XRD study, a second phase assigned to Sr2CeO4 was observed under strongly reducing conditions such as dry H-2 with P(H2O)=4.6 x 10 Pa [ P(O-2)=3.1 x 10(-15) Pa] at 1273 K. From Raman spectroscopy, it was found that treated specimens under hydrogen atmosphere exhibited additional Raman bands, which may be assigned to the second phase. The relative intensity of the additional Raman band at 460 cm(-1) drastically increased with decreasing P(H2O) in the region P(H2O)<7.0 x 10(2) Pa (2 degreesC sat.), whereas it was quite low in the region P(H2O)>7.0 x 10(2) Pa [P(O-2)>3.0 x 10(-14) Pa] at 1273 K. The perovskite phase proved to be stable against hydrogen with an increase in P(H2O) or a decrease in temperature. (C) 2004 Elsevier B.V. All rights reserved.