This work experimentally studies sorption of nitrogen on natural and thermally processed samples of Latvian illite clays with a particle size of less than 63μm which originated from 4 deposits – Tūja (Devonian period), Ceplīši and Pavari (Quarternary period) and Strēļi (Jurassic period). The specific surface of the obtained samples was measured using BET method from low temperature (77 K) physical nitrogen adsorption-desorption, measured by a volume method on the specific surface and measured by a porosity size analyzer Quadrasorb SI (Quantachrome USA). Optimal conditions of thermal treatment of the natural clay samples were determined. The impact of thermal treatment on natural clay samples was analyzed. It was defined that with the increasing thermal treatment temperature of clay samples up to 3000C, the specific surface increases, which can be explained by the elimination of the physically bonded water from their surface and the release of mesopores. With the increasing temperature of the clay samples thermal treatment from 300 to 500 0C, specific surface of clay samples reduces which can be explained by the beginning of the phase transition. With the increase of the temperature of the clay samples’ thermal treatment from 500 to 700 0C, the specific surface of the samples is reduced significantly because of the release of crystalline bonded water and the beginning of the particle sintering process. With the increase of the temperature of the clay samples’ thermal treatment up to 900 0C there occurs the clay particle sintering, and so their specific surface is the smallest one comparing to the other samples, which were processed at lower temperatures. The obtained results contain important information about specific surface and sorption properties of Latvian clays, serving as an addition to the database of Latvian clay deposits which have not been studied before.