A numerical study of the fluid-chemical interactions in a steady and time-varying laminar non-premixed jet flame was conducted to advance understanding of the complex interplay between the flame chemistry, fluid dynamics and soot evolution. Modelling of the steady flame is performed with two alternative reduced mechanisms and compared with the significant body of experimental data that are now available to provide confidence in the calculated values of mixture fraction, which was not previously available. A Method-of-Moments soot model with a 47-species mechanism provides much better agreement with the measured soot volume fraction than does a 32-species mechanism, but both mechanisms predict both the temporal and spatial profiles of mixture fraction to agree within 6%. Nevertheless, neither scheme predicts a reduction in temperature that coincides approximately with the location immediately upstream from the measured soot field. The calculations of the unsteady flame also reveal new insights about the cause of the pinch-off point and the neck zone, together with the role of buoyancy at the flame tip. (C) 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.