Prob.(I): Thermo diffusion and chemical effects on heat transfer in MHD mixed convection flow and mass transfer past an infinite vertical plate with Ohmic heating and viscous dissipation have been studied. Thermal diffusion caused both the fluid velocity and temperature to fall due to the presence of the chemical effect. Velocity and temperature profiles are higher for mercury than electrolytic solution. Soret effect increased the concentration of the fluid while chemical effect decreased. Prob.(II): A three - dimensional MHD flow with heat and mass transfer through a semi-infinite porous medium, in the presence of viscous dissipative heat and chemical reaction, is considered. A uniform magnetic field is applied normal to the bounding surface. Velocity of the fluid is increasing with velocity ratio parameter, Hartman number and Reynolds number whereas decreases with permeability parameter. Increase in Prandtl number and Reynolds number led to increase in temperature of the fluid. Fluid concentration decreased with increase in Chemical parameter and Schmidt number.