An a priori analysis of a recent formulation of the conditional-moment-closure (CMC) model is carried out using results of a 3-D direct numerical simulation (DNS) with reduced ethylene chemistry and a simplified soot model. Of particular interest is a term that is new in the recent CMC formulation. This term is associated with the role that differential diffusion plays in transporting soot relative to the mixture fraction coordinate. In particular, it describes the role of small-scale diffusive processes on that transport, and it has been modeled using an eddy-diffusivity approximation. The results suggest that the eddy-diffusivity approximation works well over a wide range of conditions for soot, but that the approximation, in the present form, breaks down for species for which the flame chemistry is fast, such as those participating in the main-flame chemistry. This breakdown is analyzed in a way that suggests a correction to the eddy-diffusivity model for fast chemistry, but this correction has not been carried out at this point.