Oil shale is a low-grade fossil fuel that has been used in Estonia mostly for electricity and shale oil production. The conventional use of oil shale results in a vast amount of carbon dioxide emissions urging to find alternative ways for sustainable utilisation of oil shale and develop new technologies. Since oil shale contains relatively high amount of organic matter it should be suitable for producing porous carbonised material (semi-coke) that has high surface area and can be used as an adsorbent in various applications. The aim of this research is to determine the effect of thermal treatment conditions on the specific surface area (SSA) of semi-coke. Estonian oil shale from Ojamaa underground mine was pyrolysed in a batch reactor using different temperatures in the range of 550 to 900 °C and atmospheres (N2 and CO2). The surface properties (BET SSA, pore volume and pore size distribution) of obtained samples were studied using N2 adsorption. The results showed that when the pyrolysis temperature was raised in N2 environment, larger SSA of the sample was obtained – it increased from 12 m2/g (pyrolysed at 550 °C) to 31 m2/g (pyrolysed at 900 °C). At higher temperatures the increase of pores less than 5 nm in diameter was also noticed. The semi-coke samples prepared in N2 environment had larger SSA compared to the ones prepared in CO2.