A detailed experimental study was performed in a rapid compression machine to investigate the effect of several EGR and syngas components on the autoignition of natural gas. Ignition delays of methane, natural gas, and mixtures of natural gas with hydrogen, carbon monoxide, carbon dioxide, and water were measured for temperatures ranging from 870 to 1000 K and pressures from 18 to 24 bar, at equivalence ratios of 1 and 0.7. The results show an accelerating effect of H-2 blending. The experimental measurements are compared with results from numerical simulations using detailed chemical mechanisms from the literature. Natural gas ignition delays can be reproduced satisfactorily, but improvements need to be made to give an accurate, quantitative account of the effect of the inclusion of hydrogen.