Mixed perovskites (AA')(BB')O-3, where A = La or Nd, A' = Sr, B = Al or Ga, and B' = Ta or Nb, were investigated by X-ray photoelectron spectroscopy (XPS), The interpretation of XPS C(Is) signals in mixed oxides is discussed. The nonsingular C(Is) peak that occurs in the mixed oxides is due to the polarization of the oxygen valence electron density, as was already proposed by us for Y(3)AL(5)O(12) and YAlO3. This interpretation seems to be true also for the case when four different metals are in the oxide. The C(ls) binding energies seem to depend on the direction in which they are ejected.