A computational model for the mass transport occuring during the conversion of GaAs to GaAsP on a GaP substrate is presented. The mass transport equations in the liquid and solid phases, and the equations describing phase diagram constitute the governing equations. These equations together with appropriate interface and boundary conditions were solved numerically by the finite element method. Numerical solutions agree with experimental results and explain well the conversion phenomenon. The conversion process is initiated by the non-equilibrium condition between the GaAsP solution and the GaAs layer and promoted by the rapid diffusion of V elements into the GaAs (GaAsP conversion) layer.