High quality GaN epitaxial films have been grown on sapphire using iodine vapor phase growth (IVPG). The growth mechanism has been investigated to determine the effect of pressure, flow rate, and V-III ratio on the crystalline quality of GaN epi-lavers. A major distinction between this process and conventional hydride vapor phase epitaxy (HVPE) is the replacement of HCl gas by iodine as a reactant. Ammonia is the source of nitrogen, and forming gas or hydrogen is used as carrier gas, C-plane sapphire wafers with pre-deposited 1 mum MOCVD films were used as substrates. Growth conditions were investigated in the range of pressure between 1 and 0.1 atmosphere, while maintaining the same temperature gradient and mass flow rates for the reactant gases. Within these parameters, it was found that the fastest growth rate for GaN layers is obtained at vapor pressures close to equilibrium. The data indicates crystal quality is also optimized under these conditions. Crystal quality was determined from the FWHM of X-ray rocking curves and room temperature photoluminescence (PL). The crystal quality was found to improve under conditions where the ambient pressure and supersaturation approach equilibrium. Published by Elsevier Science B.V.