Preignition-limited performance data were obtained on a supercharged CFR engine at two sets of operating conditions over a wide range of fuel-air ratios to determine the preignition characteristics for the following five fuels S-3 reference fuel, S-3 plus 4 ml TEL per gallon, AFD-33 140-P, benzene, and diisobutylene. Maximum thermal-plug temperatures at constant intake-air pressures were also determined to correlate the preignition characteristics of each fuel with its ability to increase general engine-temperature levels. Additional runs were made to compare the preignition-limited performance of triptane, triptane plus 4 ml TEL per gallon, and AN-F-28R fuel. The results indicated that in the fuel-air-ratio range from 0.070 to 0.085, the relative order of the fuels according to increasing preignition-limited indicated mean effective pressures was diisobutylene, benzene, S-3, AFD-33, and S-3 plus 4 ml TEL per gallon. The relative order of the fuels for the two sets of operating conditions was not the same for all fuel-air ratios. The addition of tetraethyl lead to either S-3 reference fuel or triptane raised the preignition-limited performance. The plot of fuel flow against air flow at the preignition limit for each fuel shows that the data approximately follow two straight lines one for the rich-mixture region and one for the lean-mixture region. The respective slopes of these lines changed only slightly in the various preignition-limited runs.