An investigation was carried out into the differing sooting tendencies of various fuels in free round laminar diffusion flames. Measurements included soot concentrations, soot temperatures and flame reaction zone and centreline temperatures. It is shown that flames emit smoke when the soot temperature in the oxidation zone falls below about 1300 K. This temperature is controlled by heat losses through radiation from the soot formed. Measured flame temperatures are well below adiabatic temperatures in the sooting flames and are determined by fuel flow rate and radiation losses. Thus acetylene and butene produced the highest soot yields at the lowest flame temperatures for the fuels tested. Fuel dilution with N 2 reduced soot yield more than could be explained by variation of the flame temperature. Carbon to hydrogen ratio of the fuel is not itself a parameter determining soot yeild. Soot yield profiles were normalized according to the stoichiometric flame length. Under these conditions the profiles from differing fuels showed good axial similarity except for C 2 H 2 .