This paper reports the prediction of optimal conditions to grow good quality crystalline thin films using the Close-Spaced Vapor Transport process. A new configuration of the horizontal reactor is used and presented. A thermodynamic model is proposed for the Cu-In-S-Se-I system to describe the deposition of CuIn(S0,4Se0,6)2(CISS) compound. The simulation was performed using the SOLGASMIX software which gives the composition of the chemical system at the thermodynamic equilibrium. The model is based on the minimization of the Gibbs energy of the defined chemical system. The present study has allowed us to determine the influence of the source temperature (TS) and iodine pressure (PI2) on the growth of CISS thin films. The different compounds of the solid phase were predicted for various TS and PI2 values. The conditions of stoichiometric and quasi-stoichiometric deposition are 475 ≤ TS ≤ 525 °C and PI2 ≤ 3 kPa. Some deduced conditions from the theoretical prediction were tested experimentally. The CISS samples grown have been analyzed by X-ray diffraction and scanning electron microscope. The thin films, deposited in optimal conditions, are stoichiometric.