Soot particulates from diesel engines cause serious environmental and health problems. Catalytic combustion removal of soot by alkali metal oxides is one of the most promising aftertreatment techniques because no precious metals are used. However, the activation mechanism of gaseous oxygen and/or soot by alkali metal oxides remains unclear. Here, Cs-supported typical oxides were reported to exhibit superior catalytic performance for soot combustion to the most studied K-supported counterparts. The active species were characterized as Cs2O, whilst ketene was still distinguished as the reaction intermediate as in the case of the K-supported catalysts. The easier electron donation from Cs2O to O-2 than from K2O results in the more active oxygen species with higher charge density. On the other hand, soot activation by chemisorption on the Cs2O surface is easier than on the K2O surface via the oxygen-containing group at the edge of soot, leading to more electrons donated from Cs2O to soot. These theoretical calculation results were further confirmed by in situ IR of soot combustion. The understanding of the electron donation mechanism provides an insight into the essence of soot combustion catalyzed by alkali metal oxides.