The diffusion process in multicomponent alloys has a significant influence on the evolution of the microstructure. The Calphad approach is a powerful method for describing the equilibrium state as well as the kinetics of non-equilibrium systems via the Gibbs energy. In this work, the principles of multicomponent diffusion theory are considered intensively, and an equation for the fluxes in the case of substitutional-interstitial diffusion is given for implementation. Additionally, the calculation of mobility matrices and thermodynamic factors is addressed. As an application case, substitutional diffusion is implemented in OpenCalphad and is used for calculating the growth rate for spherical precipitates from a supersaturated aluminum matrix. The growth rate has been integrated into the Kampmann–Wagner numerical model, which describes nucleation, growth, and coarsening for spherical precipitates. A AlMgZnCu alloy is considered, which has great significance in the field of materials processing.