A new series of mixed oxide superconductors with the stoichiometric composition La2−x Dy x Ca y Ba2Cu4+y O z (x=0.0 − 0.5, y=2x) has been studied for structural and superconductiong properties. Our earlier studies on La2−x (Y/Er) x Ca y Ba2Cu4+y O z series, show a strong dependence of T c on hole concentration (p sh). In the present work, the results of the analysis of the neutron diffraction measurements at room temprerature on x=0.3 and 0.5 samples are reported. It is interesting to know that Ca substitutes for both La and Ba site with concomitant displacement of La onto Ba site. Superconductivity studies show that maximum T c is obtained for x=0.5, y=1.0 sample (T c ∼ 75 K), for La1.5Dy0.5Ca1Ba2Cu5O z (La-2125).