In order to determine blending sensitivity, lead susceptibility, and sensitivity of the blends to inlet-air temperature, knock-limited small-scale-engine tests were made of toluene, ethylbenzene, p-xylene blended individually in various concentrations with selected base fuels. Toluene, ethylbenzene, and p-xylene increased the knock-limited indicated mean effective pressures of the two base fuels from 5-183 percent at high fuel air ratios depending on operating conditions and percentage of aromatics. At lean fuel air mixtures the addition of the aromatics increased the knock-limited indicated mean effective pressure of the base fuel 10-39 percent in the full-scale cylinder and 0.33 percent in the 17.6 engine. The general trend indicated that the lead susceptibility of the aromatic blends increased with increasing aromatic content and decreased with increasing inlet-air temperature.