Shock-tube ignition delay time data have recently been called into question on the basis of possible hydrocarbon impurities that may serve to artificially accelerate ignition delay times. To assess potential sources of impurities, systematic tests were performed in highly dilute H₂/O₂/Ar and CH₄/O₂/Ar mixtures using H₂O laser absorption at 1.388 µm and OH* emission at 307 nm near 1 atm and between 1257 and 2108 K. Factors investigated included common sources of impurity, namely shock-tube cleanliness, Ar purity, diaphragm fragments, leftover cleaning substances, turbopumping duration, and mixing tank cleanliness, but only mixing tank cleanliness was found to have any quantifiable effect on measured ignition delay times. Rate measurements of H + O₂⇆OH + O using H₂O time-histories were found to be unperturbed by impurity effects and were in excellent agreement with the Hong et al. measurement. Mixing tank impurities were found to perturb ignition delay times of a stoichiometric H₂/O₂ mixture in 98% Ar but had no effect on a stoichiometric CH₄/O₂ mixture in 99% Ar. Even with clean mixing tank conditions, H₂/O₂ ignition delay times in 98% and 99% Ar displayed a 25–30% discrepancy with predicted values but showed remarkable agreement with two sets of historical data. This discrepancy was modeled by including trace hydrocarbons (within certified purity levels) in simulations. For less-dilute (≤ 94% Ar) H₂/O₂ mixtures, literature data seem to agree rather well and were insensitive to trace hydrocarbons according to simulations. Furthermore, experimental and modeling evidence strongly suggests that almost every hydrocarbon C1 and higher is insensitive to impurities with respect to ignition delay times. It is thus concluded that the only data for which an impurity effect seems to exist is for highly dilute (98–99% Ar) H₂/O₂ data, while H₂/O₂ data below 94% dilution and almost every hydrocarbon C1 and greater are insensitive to common impurities.