Numerical simulations are carried out to study the simultaneous effects of thermal and concentration diffusions on a mixed convection boundary layer flow over a permeable horizontal flat plate with suction/injection in a viscous incompressible fluid. The non-linear coupled partial differential equations governing the flow, thermal and concentration fields are first transformed into a set of non-linear coupled ordinary differential equations by a set of suitable similarity transformations. The resulting system of coupled non-linear differential equations is solved using shooting method by converting into initial value problem. In this method, system of equations is converted into a set of first order system which is solved by fourth order Runge-Kutta method. Flows with both assisting and opposing buoyancy forces are considered in the present investigation. The study reveals that dual solutions of velocity, temperature and concentration profiles exist for certain values of suction/injection and buoyancy parameters. Suction/injection parameter, Prandtl and Schmidt numbers strongly affect thermal and concentration boundary layers, respectively. (C) 2012 Elsevier Ltd. All rights reserved.