The possibility of obtaining high efficiency and low emissions of hazardous compounds by using decomposed methanol, i.e., hydrogen and carbon monoxide, as a fuel for spark-ignition engines has been investigated theoretically and experimentally. A cycle simulation predicts that the most important advantage with the system is the opportunity to run the engine on lean air/fuel mixtures.In the experiments, bottled gas was used instead of gas from a decomposition reactor. The results indicate that the efficiency gain, relative to neat methanol, is 15-20% in the torque interval 10-60 Nrn. Detected NO.1 emissions are very low. A control system for the on board decomposed methanol engine has been developed. which uses engine speed as the governing parameter.