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 “EFFECT OF CATIONIC SUBSTITUTION ON THE STRUCTURE AND MAGNETIC PROPERTIES OF A2FeCoO6 (A = Ho, Dy, Tb, Eu and Sm)”, 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.