Earlier it has been shown that the method for calculation of the adsorption isotherm used at adsorption of non-polar low-molecular organic molecules [1] can be applied for description of adsorption equilibrium of the systems fluoride - pore aluminum oxide sorbents [2, 3]. The use of the approach in the calculation of adsorption isotherm and description of adsorption equilibrium in the system diatomite-fluorine is interested as from theoretical as the practical view points and particularly, for the elaboration of the technology of purification of water from toxic components. In the work there are shown calculation of adsorption isotherm of fluorine on diatomite. The calculation of Ka2 constant is based on the law of distribution of adsorbed component NaF between two phases – solid and liquid ones. The binary system has been examined in which H2O molecules were the first component and the molecules of dissolved NaF were the second one. The fluorine adsorption experiment has been carried out in agitated batch system with the initial concentration of the model solution of NaF 0.05-0.15mol/L at pH 5.5, at 380 rpm, adsorbent dose – 4g/L, the diatomite surface area - 35.5m2/g, adsorption pore volume - 0.143cm3/g. Adsorption values have been calculated with taking into account the activity coefficient of NaF molecules in the adsorption layer and in the bulk solution. For the isotherm adsorption obtaining it is necessary to know the Ka2 which has been determined according to the equation: formulawhere Θ2 – the NaF molecules surface covering degree which is equal to m v a v 0 2 2 ⋅ where a2 - the adsorption value, mmol / g, vm- the sorbent maximum adsorption pore volume cm3/g, v2 0- mole volume of NaF, cm3 / mmol, C2 - NaF solution equilibrium concentration, mmol / L, γ2 and γa2 - the NaF molecules activity coefficients in the equilibrium bulk solution and in the adsorption layer, respectively. At the fluorine adsorption the following assumptions have been made: the absence of the interaction between NaF adsorbed molecules with each other and with water molecules in the adsorbed layer. Adsorption isotherms have been calculated as follows: for given values Θ2 which is proportional to a2 conform equation (1) the equilibrium values of NaF concentration in the solution, C2, have been calculated. The activity coefficients of Naf molecules, γa2, γ2 determined conform [3] have been used at the calculation. The adsorption values, a2calc calculations have been performed by equation: а2calc. = Θ2 am, where am- the maximum adsorption, mmol/g which is calculated from the relation:Then the isotherm has been constructed in coordinates а2 vs С2 and compared with experimental determined adsorption values a2exp. Calculated in such a way adsorption capacity of diatomite toward the fluorine has been compared with the experimental adsorption data at the same equilibrium concentrations of fluorine in the solution. Good correlation of the experimental and calculated adsorption isotherm has been shown.