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Peat generally found in thick layers in limited areas such as Urmia Lake, Iran has low shear strength and high compressive deformation. Therefore, an appropriate soil improvement method is required. This paper presents an experimental study of the stabilisation of Urmia Lake peat on the Urmia–Tabriz highway, 7·8 km to the east of Urmia city in Iran. A type of fly ash (artificial pozzolan) and two types of volcanic ashes (natural pozzolans) were used in this investigation. Unconfined compressive strength (UCS) and California bearing ratio (CBR) tests were conducted on stabilised mixtures and untreated soil samples after 3 and 7 d of curing. According to the obtained results, the UCS of peat can be increased by using both the fly ash and the volcanic ash, but the amount of strength increase depends on the organic content, water content, stabiliser content and its particle size. Furthermore, the CBR test showed that the filling property of the stabiliser is as useful as its pozzolanic activity in increasing the bearing capacity of peat in road constructions.