In this work the dissolution kinetics of pure CaO, lime and dololime containing various amounts of magnesium oxide in steelmaking slags has been studied in a non-steady-state diffusion, at temperatures ranging from 1300-1600°C, directly in the hot process. Experiments were carried out in stagnant regime and under forced convection. In the frames of the K1-Met project - the member of COMET – Competence Centers for Excellent Technologies, the standardized method for the study of kinetics of lime and dololime dissolution in the converter slag has been developed. To control and improve the steelmaking process it is important to know the rate-limiting step of lime dissolution in the melt in the process of interaction between metal and slag. To describe the kinetics of metallurgical processes physico-mathematical method based on fundamental laws of physical chemistry was applied. Using differential equations of mass balance of the dissolving substance values of mass transfer coefficients were determined experimentally and allow estimating the rate of conjugated mass transfer processes or chemical reactions. Pure CaO dissolved more rapidly and of bigger mass compared to industrial limes. Calculated apparent activation energy value for the dissolution of pure CaO and lime in the slag is less than 200 kJ/mol, which characterizes the diffusion limiting step in the process. The dependence of the linear rate of lime samples dissolution on the saturation concentration of CaO in the slag has been derived, indicating the diffusion regime of the CaO dissolution in the melt. It was revealed that the decrease in solubility of lime with the increasing content of magnesium oxide in lime is due to the formation of high temperature solid chemical compounds (Сa2SiO4, (Mg, Fe)2SiO4) and solutions based on MgO like ((Mg, Fe) O) on the surface and in the volume of samples. Slightly soluble compounds formed at the interface of liquid slag-lime hinder the process of lime dissolution. Using non-stationary linear and spatial diffusion models on the basis of experimental data diffusion coefficients and diffusion activation energy were calculated. It was found that the rate limiting step in the process of lime dissolution in slag is the diffusion of MgO in the melt for slag systems containing high amount of FeO. Dicalcium, tricalcium silicates and magnesiowustite formed in the process of lime dissolution envelop the dissolved lime particles and limit the diffusion through a solid layer, where the rate-limiting step of the process is internal diffusion.