Prediction of grain boundary cementite growth kinetics in hypereutectoid steels is a question of major industrial importance. Mechanical properties of the alloy deteriorate with greater cementite thickness. New modelling approaches were recently suggested for significantly improved simulation of grain boundary cementite thickness over time in steel with up to three elements [1]. In the present work, we present chemical analysis of grain boundary cementite in multi-component hypereutectoid steels at different stages of isothermal cementite growth, using Electron Probe Micro Analysis (EPMA) data. These experimental results are then compared to simulations of cementite precipitation using a modification of SFFK model within MatCalc [2], which can account for heterogeneous site nucleation energy [1]. The model parametrization for complex steels is validated by comparison of the predicted cementite thickness with experimental results. Parameters include an assessment of the multi-component thermodynamic equilibrium and the diffusion mobilities database (Matcalc tdb and ddb databases), the choice of the cementite precipitate character (para- or ortho-equilibrium), and the consideration of diffusion to and within grain boundary cementite (this is particularly relevant at long reaction times). The discussion aims on the solution to the question: how can we model correctly the influence of alloying elements on grain boundary cementite growth in complex systems? References: 1. M. Vogric, E. Kozeschnik, J. Svoboda, M. Führer, J. Kreyca, W. Wei, E. Povoden-Karadeniz: “Kinetic modeling of grain boundary cementite evolution”, Metall. Mater. Trans. A, under review. 2. E. Kozeschnik: “Mean-field microstructure kinetics modeling”. In: Francisca G. Caballero (ed.), Encyclopedia of Materials: Metals and Alloys 4 (2022) 521-526.