Soot measurements are made in long laminar ethylene diffusion flames to determine the effect of long residence times. Flames with lengths greater than 1 m are obtained by stabilizing them in a round grid. Although soot volume fraction distributions for the long flame are different to the shorter flames, the maximum conversion fraction of fuel into soot remains about the same for the whole range of flame sizes. An important finding is that the total soot mass flow (kg/s) at any height in the soot growth region of the flame is only a function of the particle trajectory time from the reaction zone. This is true irrespective of the flame length and relative position in the flame. The surprising result implies that soot particle growth rates averaged across the flame section are not dependent on the gas-phase environment.