Abstract The effects of partial premixing on soot formation were investigated both experimentally and numerically in counterflow diffusion flames (CDFs) of ethylene, propane and ethylene/propane binary mixtures. The partial premixing ratio was maintained at relatively low levels so as to minimize the hydrodynamic effects; while the chemical effects of partial premixing on soot formation can be highlighted. The experimental results and simulations consistently showed that partial premixing notably increased soot formation in ethylene flames; while it slightly reduced the soot formation in propane flames, suggesting that partial premixing played different chemical roles in the soot formation process of ethylene and propane flames. Kinetic analyses revealed that partial premixing showed opposing effects on propargyl formation of ethylene and propane flames; this in turn led to the opposing effects on the formation of benzene and polycyclic aromatic hydrocarbons (PAH), and finally on the soot inception process. For partial premixing in propane/ethylene mixtures, the observed synergistic effects with propane added to ethylene CDFs disappeared, indicating a chemical cross-linking effect between partial premixing and ethylene/propane mixture. In addition, the present soot model showed satisfactory performance in reproducing the mismatch between PAH and soot formation tendencies in ethylene and propane CDFs. This result implies that great care is needed when using PAH concentration as an indicator of sooting tendency of different fuels. The present modeling results provided new insights into the chemical role of partial premixing in the soot evolution process of ethylene, propane and their blends. Further investigations towards the partial premixing effects on soot formation of larger liquid hydrocarbons such as diesel surrogates are required.