In this work, a new approach to mercury speciation study on solid samples was tested and verified. This approach is different from commonly applied sequential extraction techniques. Mercury speciation was expressed as a stability constant K, describing the bonding equilibrium between mercury and solid. Stability constant K was measured on soil sample from Příbram, which was characterised in detail (chemical analysis, powder XRD diffraction, thermo desorption analyses). Using solution of potassium resp. ammonium bromide, iodide and thiocyanate the K value was quantified in a wide concentration range. We showed that in mercury in studied sample was present in a single specie with pK  36. We discussed the effect of extractant concentration and experimental setting on resulting K. The result of extraction experiments was in accordance with thermo desorption results confirming mercury present as one specie bound onto clay matrices. As a profound difference in approach to common speciation extraction techniques, in which a set of operationally defined mercury fractions is obtained, the K value obtained in this work represents a thermodynamical parameter feasible for further mathematical processing. Thiocyanate was identified as a useful anion for extraction studies of mercury speciation on solid...