This paper aims to examine the effect of hydrogen purity on the combustion, performance, NOx emissions and energy costs in a spark ignition engine. In accordance with this purpose, two commercial hydrogen gases of different purity (i.e. 99.998% and 99.995%), were used as fuel in an spark ignition engine. The engine was operated under a lean mixture (phi = 0.6) and wide-open throttle conditions at 1400, 1600, and 1800 rpm engine speeds. It was found that high purity hydrogen improves engine performance parameters (i. e. indicated power, torque, thermal efficiency, and specific fuel consumption) in the range of 2.4-1.9% depending on engine speed. The combustion duration and the cyclic variations were also decrease when the engine is operated with high purity hydrogen. However, NOx emissions increase depending on engine speed in the range of 3.4-2.9% when high purity hydrogen is used as a fuel. In addition, energy costs with high purity H-2 increase in the range of 5.9-6.5% depending on engine speed.