Double perovskite oxides (A2BB'O6) with first-row nearest neighbour transition metals at Bsites are rarely studied, even though they exhibit unusual properties. The thesis entitled â€œEFFECT OF CATIONIC SUBSTITUTION ON THE STRUCTURE AND MAGNETIC PROPERTIES OF A2FeCoO6 (A = Ho, Dy, Tb, Eu and Sm)â€, presents the synthesis, structure and magnetic characterizations of double perovskite oxides A2FeCoO6 bearing nearest neighbour cations (Fe, Co) at B-sites. Sol-gel synthesized compounds are assigned orthorhombic structure by Rietveld refinement based on the data obtained from room temperature X-ray diffraction experiments. Unit cell parameters reflect variations with respect to the change in A-site, owing to the lanthanide contraction along the periodic table. The compounds exhibit a variety of magnetic phenomena such as spin re-orientation, magnetization reversal, antiferromagnetism, negative magnetization, exchange bias and bipolar switching due to the coexistence of competing magnetic interactions.