Soot surface reactivity is numerically studied in laminar ethylene/air and methane/air coflow diffusion flames. Surface reactions are found to be important for many processes involved in soot formation, including surface growth and oxidation which contribute significantly to soot yield. It has recently been shown that soot particle reactivity changes as particles age during both surface growth (Veshkini et al., 2014) and oxidation (Khosousi and Dworkin, 2015). A newly developed surface character model simultaneously accounts for soot surface reactivity in surface growth and oxidation by considering soot ageing and its effects on the particle surface, reconciling the aforementioned works. This new development eliminates tuning from case to case of one parameter used in soot numerical simulations, alpha, the portion of soot surface sites available for reaction, which is implemented as a function of temperature and residence time (particle history). Thus, the new model reconciles the quantification of the evolving soot surface character for both growth and oxidation. The model is shown to be uniquely capable of predicting soot concentrations and smoke emissions within experimental uncertainty in a wide range of laminar diffusion sooting flames, without any variation of model parameters. (C) 2015 The Combustion Institute. Published by Elsevier Inc. All rights reserved.