Critical Rayleigh numbers determined by linear stabiliy theory are presented for porous-fluid layers of infinite horizontal extent heated internally by a uniform volumetric energy source in the fluid. The thermal coupling between the layer and its environment is represented by a general mixed boundary condition for both the conduction state and the disturbance temperature. Rigid-rigid, rigid-constant pressure, and constant pressure-rigid boundaries are considered in the computations. For a fixed ratio of upper surface Biot number to that at the lower surface, decreasing the Biot number is strictly destabilizing for values of this ratio greater than or equal to one. A layer with a rigid upper surface is generally the most stable; however, a layer with a rigid upper surface and a constant pressure lower surface exhibits the largest values of critical Rayleigh numbers for large values of Biot number.