An original phenomenological model for soot production inside a laminar, flat plate boundary layer diffusion flame is presented. The model is compared with experimental measurements conducted in microgravity. For the experiments, the fuel, ethylene, is injected through a flat porous burner into an oxidizer stream flowing parallel to the burner surface. The oxidizer is a mixture of 35% oxygen and 65% nitrogen. The fuel and oxidizer velocities are systematically varied. The analysis of the data shows that the streamwise location of the maximum flame height can be considered an unambiguous characteristic length of the flame as opposed to the maximum visible flame length. Analysis of the streamwise location of the maximum flame height enables to establish the transition between “open-tip” and “closed-tip” be- havior as well as scaling laws for the soot volume fraction. A scaled soot volume fraction is found to follow a linear relationship with the streamwise coordinate normalized by the burner length. This correlation appears to be valid for the whole range of conditions investigated, knowing that this range does not cover the blow-off regime.