[en] The substitutional effect of Mo on the magnetic and transport properties of double exchange ferromagnets, La_0.5Sr_0.5Co_1-x Mo _x O₃ (0≤x≤0.2) has been investigated. Substitution of 10% Mo at the Co-site of La_0.5Sr_0.5CoO₃ decreases the Curie temperature by ∼60 K than that of the parent compound and the long-range ferromagnetic ordering disappears for x≥0.2. The Mo-doped samples, however, undergo a transition from the parent metallic state to the insulating state below T _c. The insulating state is found to obey Mott's variable range hopping of conduction. The effect of Mo substitution is attributed to the factors namely (i) the dilution of magnetic Co sublattice (ii) the reduction of Co⁴⁺/Co³⁺ ratio resulting in a reduced carrier concentration and (iii) disruption of the intermediate spin structure of Co, namely Co³⁺: t_2g⁵e_g¹