It has been noted for some years that the concentrations of many species in laminar hydrocarbon diffusion flames correlate with mixture fraction, or alternatively, local equivalence ratio. Therefore, once the spatial profile for the mixture fraction is established, it is possible to approximate both the concentration and net chemical rate profiles for a great many flame species. However, some species exhibit concentration gradients along contours of constant mixture fraction in a flame. The results of our past work show that most of the species along the chemical pathway leading to soot particle formation in diffusion flames, including all of the polynuclear aromatic hydrocarbons, exhibit this type of behaviour. For these species, it is necessary to consider not only the chemistry of the growth environment, which may be adequately described by the mixture fraction, but also the residence time within the growth region. This paper will describe how such a model could be expressed, and present some initial comparisons with laboratory flame data.