Onset of double diffusive convection in a horizontal fluid saturated porous layer which is subjected to concentrationbased internal heat source is studied. Here the flow is induced by vertical temperature and concentration differences across the boundaries. Effect of vertical throughflow is also considered in this investigation. The flow-governing parameters involved in this study are thermal Rayleigh number RaT and solutal Rayleigh number RaS, which are associated with thermal and concentration differences in the vertical direction, along with the Peclet number Pe, Lewis number Le, and measure of concentration-based internal heat source γ. To perform linear stability analysis, an eigenvalue problem is derived for Rayleigh number RaT, and it has been obtained numerically. Critical thermal Rayleigh number RaTC is evaluated for various values of flow-governing parameters. It has been noticed that oscillatory longitudinal modes are preferred modes of instability. © 2016 by Begell House, Inc.