Samples of swelling soils from sites in Rhodope Peripheral Unit (former prefecture) were stabilized using lime and fly ash and the alteration of engineering properties such as the plasticity, the relation between moisture and density and the compressive strength has been investigated in the laboratory. It is established that lime is a very good stabilizer, acting beneficially not only on the strength characteristics, but on the optimum moisture and maximum dry density as well. The pozzolanic properties of the fly ash are mainly responsible for the improvement coming upon the characteristics of the tested soil materials. The role of specimen’s curing time is important, more specifically in the case that lime is used as additive. By investigating a large number of soil materials, it is feasible to establish suitability criteria for their use in stabilized subgrade and roadway layers.