Based on the trapping concept [1,2], a thermodynamic model describing the Gibbs energy of the system accounting for Mg-Si dimer formation in an Al-Mg-Si matrix is developed. The equilibrium concentration of dimers is obtained by minimizing the Gibbs energy of the system for fixed temperature, chemical composition and the trapping energy describing the energy gain due to Mg-Si dimer formation relatively to monomers of Mg and Si atoms. The evolution equations describing the kinetics of dimer formation are derived by means of the Onsager thermodynamic extremal principle. The kinetics is deue to superposition of two processes: i) Mg atoms are considered as immobile traps and Si atoms can diffuse and fall into the traps, ii) Si atoms are considered as immobile traps and Mg atoms can diffuse and fall into the traps. The studies based on this model, can significantly contribute to a better understanding of the early stages in nucleation of complex precipitates. The predictions of our model for equilibrium states are compared to similar approaches in open literature, such as the model of Howard and Lidiard [3], which uses a different approach to the treatment of the configurational entropy and provides different results.