The amounts of chloroform-soluble material, polycyclic aromatic hydrocarbons (PCAH) and cvrbonaceoas residue in the dry soot recovered from ethylene-air and ethane-air diffusion flames respectively have bcea determined and t he effects on these of incseasing additions of oxygen to the fuel have been investigated. With ethylene, the amounts of dry soot, chloroform-soluble material, PCAH and carbonaceous residue (CR) all increase to maxima and then decrease. With ethane, the amounts of dry soot and CR increase to maxima and then decline whereas the chloroformsoluble material and PCAH decrease steadily with increase in oxygen/ethane molar ratio in the fuel stream. The PCAH adsorbed on the soot particles leaving the flame have been analysed by programmed temperature gas chromatography. Pyrene predominates in the mixture but the carcinogen 3,4-henzopyrene is present in srr!alles amounts. Increasing the addition of oxygen to the fuel stream is shown to lead to highar temperatures in the pyrolysis zone and an attempt has been made to account qualitatively for the above results on the basis of the effects on pyrolysis of the fuel and on the oxidation of the products Ice.ping the tip of a diffusion flame. A "qualitative model' for the formation of PCAH and *carbon" has been proposed tentatively on the basis of recent results from this laboratory and elsewhere,