The construction of a deep geological repository (DGR) for the spent nuclear fuel and high-level long-lived radioactive wastes is prescribed in Lithuanian legislation. Therefore, geological investigations for DGR host formation and site selection are being carried out. This paper presents a comparative suitability evaluation of two potentially suitable Lithuanian clayey formations. The suitability evaluation of Lower Cambrian and Lower Triassic clays was performed using the data set of laboratory test results from two decades. The results of laboratory investigations performed on clay material obtained from the unpreserved cores of nine boreholes were analysed in order to quantify the parameters relevant to the formation suitability. The data set contains the results of hydraulic conductivity measurements of several samples, mineralogical analysis of clay minerals, determination of Atterberg limits, chemical analysis (EDS method), and particle specific surface area measurements (BET method). Analysis of laboratory test results allow to compare the suitability for DGR installation of two Lithuanian clayey formations according to several properties. The clay fraction of the Lower Triassic potentially suitable formation is dominated by smectite/montmorillonite group minerals (60–88%), and the dominance of illite (60– 95%) is characteristic of Lower Cambrian clay. The determination of Atterberg limits indicates a similar (medium) plasticity of both formations. The specific surface area of particles ranges from 30.9 to 54.8 m²/g in Lower Triassic samples and from 28.0 to 32.7 m²/g in Lower Cambrian samples. A higher smectite content and a larger specific surface area show a greater swelling and sorption capacity of Lower Triassic clay. Clay swelling and sorption capacity are important features causing geological environment suitability for the DGR. Although the tests were conducted on unpreserved and degraded samples, their results support the conclusion that the suitability of the Lower Triassic potentially suitable formation is comparatively greater.