Two gases, O3 and NO2, were selected to probe the surface of a diesel fuel combustion aerosol sample, diesel soot, and amorphous carbon nanoparticles (PRINTEX XE2-B) using heterogeneous (i.e., gas-surface reactions). The gas uptake to saturation of the probes was measured under molecular flow conditions using a Knudsen flow reactor in order to quantify and characterize surface functional groups. Specifically, O3 and NO2 are used for the titration of oxidizable groups. Diesel soot samples interacted with the probe gases to various extents which points to the coexistence of different functional groups on the same aerosol surface such as reduced groups. The carbonaceous particles displayed significant differences: PRINTEX XE2-B amorphous carbon had a significantly lower surface functional group density of both total and strongly reducing groups despite its significantly larger internal surface area, compared to diesel soot. The uptake kinetics of the gas-phase probe molecules (uptake probabilities) were also measured in order to obtain further information on the reactivity of emitted soot aerosols in order to enable the potential prediction of health effects.