Condensation properties of ultrafine carbon particles in the Aitken range (particle diameter between 20 and 100 nm) were investigated by means of a variable supersaturation condensation nucleus counter. Condensation phenomena were studied with dry, monodisperse soot aerosols. For determination of the critical supersaturation, i.e. the saturation when 50% of the particles are activated, supersaturations between 10 and 200% were realized. For a quantitative description of the condensation properties, the critical supersaturation was determined and compared to the expected supersaturation, calculated according to the Fletcher theory of heterogeneous condensation on insoluble particles. For particle diameter <30 nm the experimental results were in good agreement with the Fletcher theory, whereas for larger particles deviations due to the agglomerate particle structure were observed. After reaction with ozone, activation of soot particles occurred at lower supersaturations. FTIR studies revealed that oxygen-containing functional groups are generated on the particle surface during oxidation, facilitating water uptake in the condensation process.