The spontaneous ignition of a range of alkanes (C-4-C-8) and of mixtures of the primary reference fuels (PRF), n-heptane + i-octane (2.2.4 trimethylpentane), have been investigated. Autoignition (or spontaneous ignition) delays were measured over a wide range of compressed gas temperatures (T-c) at gas densities of 131 mol m(-3) in a rapid compression machine. In the first instance, a reference temperature, which is typical of that reached in the end gas of spark ignition engines under violent knocking conditions (T-c = 900 K), was used as a basis of comparison of ignition delay of the single fuels with the comparable PRF mixtures with respect to the research octane number (RON). A broad qualitative agreement was found to exist between the ignition delay of single-component fuels and the PRF mixtures of corresponding RON, but the quantitative agreement became increasingly less satisfactory for some fuels at high RON, especially above 85. A major contributing factor appears to be that, whereas for the binary PRF mixtures quite considerable extents of reaction can occur during the course of compression, which leads to a ＇＇sensitization＇＇ of the mixture in the postcompression period at higher temperature, not all of the single-component alkanes are similarly reactive. These features are tied to the relative reactivities of the fuels and to the characteristics of the negative temperature dependence of reaction rate in the temperature range 750-850 K. However, even throughout a wide range of compressed gas temperatures, ignition delay does not appear to offer a quantitative basis for assessing the relative sensitivity of fuels to cause engine knock. It is suggested that the minimum temperature at which autoignition occurs in a given system may give a closer correlation than ignition delay for the relationship between reactivity and octane number.