A range of aliphatic hydrocarbon fuels have been burned on a porous cylindrical burner which was housed in a duct. The approach flow velocity was adjusted so that the formation of particles was suppressed and the flame turned blue. This defined a critical velocity gradient for the stagnation point flame at which the soot inception chemistry of the flame could not cope with the rate of mixing of reactants imposed by the strain rate of the flow. Measurements of the fluorescence of polyaromatic species with an excimer pumped dye laser indicated that their formation rate in flames of the lightly sooting fuels such as ethane was very sensitive to the flow field. Fluorescence measurements in a heavily sooting fuel such as propene were not as sensitive to the velocity gradient of the flow. Measurements were made of the height to the onset of soot formation in vertical, turbulent diffusion flames of the same fuels. It was found that numerical calculations of the mean scalar dissipation rates at this location in a jet were in reasonably good agreement with the values which were derived from the laminar, counterflow flame experiments.