The application of stress to multiphase solid-liquid systems often results in morphological instabilities. Here we propose a solid-solid phase transformation model for roughening instability in the interface between two porous materials with different porosities under normal compression stresses. This instability is triggered by a finite jump in the free energy density across the interface, and it leads to the formation of finger-like structures aligned with the principal direction of compaction. The model is proposed as an explanation for the roughening of stylolites -irregular interfaces associated with the compaction of sedimentary rocks that fluctuate about a plane perpendicular to the principal direction of compaction.PACS numbers: 68.35. Ct, 68.35.Rh, 91.60.Hg Morphological instabilities in systems out of equilibrium are central to most research on pattern formation. A host of processes give rise to such instabilities, and among the most intensively studied are the surface diffusion mediated Asaro-Tiller-Grinfeld instability [1], [2], [3] in the surfaces of stressed solids in contact with their melts, surface diffusion mediated thermal grooving and solidification controlled by thermal diffusion in the bulk melt [4]. In sedimentary rocks and other porous materials local stress variations typically promote morphological changes via dissolution in regions of high stress, transport through the fluid saturated pore space and precipitation in regions of low stress. This phenomenon is known as pressure solution or chemical compaction. Such processes are often accompanied by the nucleation and growth of thin irregular sheets, interfaces or seams called stylolites [5]. Stylolites form under a wide range of geological conditions as rough interfaces that fluctuate about a plane perpendicular to the axis of compression. They are common in a variety of rock types, including limestones, dolomites, sandstones and marbles, and they appear on scales ranging from the mineral grain scale to meters or greater. A common feature of stylolitic surfaces is small scale roughness combined with large vertical steps in the direction of the compression. Residual unsoluble minerals (i.e. clays, oxides) often accumulate at the interface as stylolites evolve. Despite the considerable attention given to the rich morphology of stylolites there is still no consensus on the mechanism(s) controlling their formation [6], [7], [8], [9]. Here we demonstrate that even if the stylolite is a consequence of pressure solution alone, porosity or other material property gradients may drive the roughening process. In particular, we demonstrate that a compressional load normal to a solid-solid phase boundary gives rise to a morphological instability.Generally, rocks are heterogeneous bodies with spatially variable porosities. The strain energy densities may be larger in regions of high porosity (i.e. low modulus) than in regions of low porosity. Thermodynamically, the total free energy of the system can be reduced by reducing the porosity variations. ...