This paper examines the flow stability of a polymerizing liquid in an anisotropic porous medium. The polymerization reaction leads to an increasing viscosity along the flow direction, particularly behind the advancing resin/air front, and may cause the miscible displacement process to become unstable and uneven. A linearized stability analysis of the flow within the filled region behind the liquid /air interface has been conducted. This reveals that under a moderate adverse viscosity gradient, the polymerization reaction has the potential for stabilizing the flow process by damping out disturbances of wavelengths above a critical size. Guidelines are provided for adjusting the composition of the reacting liquid and the anisotropic medium permeability to stabilize the flow process.