Oxygen stoichiometry in the perovskite system (Ba0.5La0.5) (Fe1-xCux)O3-w with x varying in the range of 0 to 1 was studied by thermogravimetry, 57Fe Mössbauer spectroscopy and different wet-chemical analysis methods. Both the maximum oxygen content and the amount of removable oxygen strongly depended on the copper-to-iron ratio. In this respect the samples containing both copper and iron were nearly identical but differed from the end members of the system. In the samples with 0.1≤x≤0.85 the maximum oxygen content varied in the narrow range of w = 0.25-0.33 whereas for the x = 0 and x = 1 samples the values of w = 0.06 and 0.60, respectively, were determined. The amount of removable oxygen (Δw) was the smallest for the Fe-free sample. It is interesting to note that, the maximum oxygen content of the x = 0.5 sample (as well as the samples with 0.15≤x≤0.85) was found to continue the trend of decreasing w with increasing RE ion size, as previously observed for the double-perovskite series BaRE(Fe0.5Cu0.5)2O5+8.