The influence of four single-ring aromatic compounds on the ignition process of iso-octane is investigated by shock-tube experiments and numerical simulations. Methylbenzene (toluene), dimethylbenzene (xylene), trimethylbenzene (without distinction of isomers), and methoxybenzene (anisole) are often used as tracers in laserinduced fluorescence (LIF) combustion diagnostics where the presence of the tracer should have a minimal influence on the engine performance. Ignition delay times for tracer-blended iso-octane/air mixtures were measured at 750-1500 K at a pressure of 40 bar, and for equivalence ratios phi of 5 and 1.0. Measured ignition delay times of the blends containing 5-10 vol.% of the respective tracer are very close to the ignition delay times of pure iso-octane/air mixtures at the same pressure, temperature and equivalence ratio. Simulations of the auto-ignition involving detailed chemical kinetics of the alkane and the aromatics predict ignition delay times that are in good agreement with the experimental results. The simulations are used for a more detailed analysis of the interaction between the alkane and the aromatics during ignition. It is shown that the tracers and the fuel are consumed at similar rates in the blended mixtures, although mixtures of pure tracer with air are significantly less reactive than the corresponding mixtures of fuel and air. Sensitivity analyses are used to investigate the coupling between the sub-mechanisms of the tracers and the fuel. The chain branching reactions of the fuel leading to the formation of OH radicals are found to be the most sensitive reactions in the pre-ignition phase. The interaction between tracer and fuel at low temperatures can be characterized as follows: The fuel initiates radical formation, and the aromatic compound accompanies the commencing reaction by using the radicals originating from the fuel for their own decomposition. In the early steps of reaction, chain branching is caused by the tracer at a slower pace compared to the fuel. Therefore, the tracer-based reaction channels merely follow the reaction progress by the pace defined by the fuel.