On the industrial scale, methanol is made by reacting mixtures of H₂, CO and CO₂ (syngas) over Cu/ZnO-based catalysts at elevated pressures and temperatures. However, the detailed reaction mechanism, and therefore the carbon source in methanol are not well understood. Based on our previous detailed density functional theory studies of various possible reaction intermediates and elementary steps in methanol synthesis on the Cu(111) surface, we developed a comprehensive microkinetic model. All model parameters (enthalpies, entropies, activation barriers, preexponential factors) are derived from first principles calculations and fitted to experimental results where necessary. This microkinetic model allows us to investigate the preferred reaction pathway for methanol synthesis, relevance of surface intermediates and the role of oxygenated byproducts at realistic reaction conditions.