Oil shale ash and semicoke, solid residues from the oil shale industry, are today disposed of separately in landfills which pose a considerable environmental hazard. In the current study, the possibility of co-depositing ash and semicoke was investigated in a small-scale field experiment. The purpose of the experiment was to elucidate which mineral changes in the landfilled material occurred and how these changes affected its permeability characteristics. For this purpose five mixtures with different ash-to-semicoke ratios were prepared and placed in the open air. Changes in the dry density, hydraulic conductivity and mineral composition of mixtures were recorded within a period of four months and after one year. During the experiment the mixtures expanded and showed increased permeability due to intensive secondary mineralization. The higher the ash content of a mixture, the more intensive the expansion and, consequently, the higher the permeability, which contributed to an increased infiltration of the leachate and toxic compounds through the landfilled material, thus leading to unfavourable environmental impacts. The above suggests the possibility of co-depositing ash and semicoke, but only if the ash content of their mixture is low enough.