In this study, central composite design (CCD) at five levels (-1.63, -1, 0, +1, +1.63) combined with response surface methodology (RSM) have been applied to optimise methanol carbonylation using a ruthenium-promoted iridium catalyst in a homogenous phase. The effect of seven process variables, including temperature, pressure. iridium, ruthenium, methyl iodide, methyl acetate and water concentrations, as well as their binary interactions, were modelled. The determined R-2 values greater than 0.9 for the rate and methane formation data confirmed that the quadratic equation properly fitted the obtained experimental data. The optimum conditions for maximum rate and minimum methane formation were obtained via the RSM to be: temperature of 191 degrees C, pressure of 32.48 barg, iridium concentration of 939.40 ppm, ruthenium concentration of 2099.22 ppm, methyl iodide concentration of 14.46 wt.%, methyl acetate concentration of 17.55 wt.% and water content of 7.60 wt.%. The results predicted by the developed correlation at the optimum determined conditions, 28.63 mol/l h for the reaction rate and 1.97 mol% CH4, were reasonably compared with the experimental data obtained for the reaction rate and methane formation of 27.10 mol/l h and 3.06 mol% CH4, respectively. Crown Copyright (C) 2011 Published by Elsevier B.V. All rights reserved.