This invention relates to combinations of (A) organometallic complexes and (B) antioxidants. These combinations can be used in diesel fuels for operating diesel engines equipped with exhaust system particulate traps. The combination of (A) and (B) is useful in lowering the ignition temperature of exhaust particles collected in the trap. The organometallic complex (A) is soluble or stably dispersible in the diesel fuel and is derived from (i) an organic compound containing at least two functional groups attached to a hydrocarbon linkage, and (ii) a metal reactant capable of forming a complex with the organic compound (i), the metal being any metal capable of reducing the ignition temperature of the exhaust particles. The functional groups include .dbd.X, --XR, --NR.sub.2, --NO.sub.2, .dbd.NR, .dbd.NXR, .dbd.N--R*--XR, ##STR1## --CN, --N.dbd.NR and --N.dbd.CR.sub.2 ; wherein X is O or S, R is H or hydrocarbyl, R* is hydrocarbylene or hydrocarbylidene, and a is a number (e.g., zero to about 10). Useful metals include Na, K, Mg, Ca, Sr, Ba, V, Cr, Fe, Co, Cu, Zn, Pb, Sb, and mixtures of two or more thereof. This invention is also directed to concentrates and diesel fuels, and to methods of operating a diesel engine equipped with an exhaust system particulate trap.